[NetBSD] virtual_memory() metrics are completely wrong (#2235), fixes #2234

Match values shown by **htop**. This is before. In here:
* available mem is almost the same as total (unrealistic)
* used is higher than total; there's also a failing test:

```
MEMORY
------
Total      :  972.7M
Available  :  959.1M
Percent    :     1.4
Used       :    1.1G
Free       :  173.6M
Active     :  434.3M
Inactive   :  258.4M
Buffers    :  509.4M
Cached     :  692.9M
Shared     :    0.0B
Wired      :  280.0K
```

Now:

```
MEMORY
------
Total      :  972.7M
Available  :  538.1M
Percent    :    44.7
Used       :  434.5M
Free       :  173.6M
Active     :  434.2M
Inactive   :  258.4M
Buffers    :  509.4M
Cached     :  692.9M
Shared     :    0.0B
Wired      :  280.0K
```

2 files changed, 35 insertions, 30 deletions

diff --git a/psutil/_psbsd.py b/psutil/_psbsd.py
index c1cc1c03..9dd8420d 100644
--- a/psutil/_psbsd.py
+++ b/psutil/_psbsd.py
@@ -176,11 +176,10 @@ else:
 # =====================================================================
 
 
-if NETBSD:
-    def virtual_memory():
-        """System virtual memory as a namedtuple."""
-        mem = cext.virtual_mem()
-        total, free, active, inactive, wired, cached, avail = mem
+def virtual_memory():
+    mem = cext.virtual_mem()
+    if NETBSD:
+        total, free, active, inactive, wired, cached = mem
         # On NetBSD buffers and shared mem is determined via /proc.
         # The C ext set them to 0.
         with open('/proc/meminfo', 'rb') as f:
@@ -189,22 +188,28 @@ if NETBSD:
                     buffers = int(line.split()[1]) * 1024
                 elif line.startswith(b'MemShared:'):
                     shared = int(line.split()[1]) * 1024
-                elif line.startswith(b'Cached:'):
-                    cached = int(line.split()[1]) * 1024
-        used = active + wired + cached
-        percent = usage_percent((total - avail), total, round_=1)
-        return svmem(total, avail, percent, used, free,
-                     active, inactive, buffers, cached, shared, wired)
-else:
-    def virtual_memory():
-        """System virtual memory as a namedtuple."""
-        mem = cext.virtual_mem()
+        # Before avail was calculated as (inactive + cached + free),
+        # same as zabbix, but it turned out it could exceed total (see
+        # #2233), so zabbix seems to be wrong. Htop calculates it
+        # differently, and the used value seem more realistic, so let's
+        # match htop.
+        # https://github.com/htop-dev/htop/blob/e7f447b/netbsd/NetBSDProcessList.c#L162  # noqa
+        # https://github.com/zabbix/zabbix/blob/af5e0f8/src/libs/zbxsysinfo/netbsd/memory.c#L135  # noqa
+        used = active + wired
+        avail = total - used
+    else:
         total, free, active, inactive, wired, cached, buffers, shared = mem
+        # matches freebsd-memory CLI:
+        # * https://people.freebsd.org/~rse/dist/freebsd-memory
+        # * https://www.cyberciti.biz/files/scripts/freebsd-memory.pl.txt
+        # matches zabbix:
+        # * https://github.com/zabbix/zabbix/blob/af5e0f8/src/libs/zbxsysinfo/freebsd/memory.c#L143  # noqa
         avail = inactive + cached + free
         used = active + wired + cached
-        percent = usage_percent((total - avail), total, round_=1)
-        return svmem(total, avail, percent, used, free,
-                     active, inactive, buffers, cached, shared, wired)
+
+    percent = usage_percent((total - avail), total, round_=1)
+    return svmem(total, avail, percent, used, free,
+                 active, inactive, buffers, cached, shared, wired)
 
 
 def swap_memory():
diff --git a/psutil/arch/netbsd/mem.c b/psutil/arch/netbsd/mem.c
index 26ab8e69..456479ba 100644
--- a/psutil/arch/netbsd/mem.c
+++ b/psutil/arch/netbsd/mem.c
@@ -31,8 +31,7 @@ psutil_virtual_mem(PyObject *self, PyObject *args) {
     size_t size;
     struct uvmexp_sysctl uv;
     int mib[] = {CTL_VM, VM_UVMEXP2};
-    long pagesize = psutil_getpagesize();
-    unsigned long long available;
+    long long cached;
 
     size = sizeof(uv);
     if (sysctl(mib, 2, &uv, &size, NULL, 0) < 0) {
@@ -40,17 +39,18 @@ psutil_virtual_mem(PyObject *self, PyObject *args) {
         return NULL;
     }
 
-    // follow zabbix
-    available = uv.inactive + uv.execpages + uv.filepages + uv.free;
+    // Note: zabbix does not include anonpages, but that doesn't match the
+    // "Cached" value in /proc/meminfo.
+    // https://github.com/zabbix/zabbix/blob/af5e0f8/src/libs/zbxsysinfo/netbsd/memory.c#L182
+    cached = (uv.filepages + uv.execpages + uv.anonpages) << uv.pageshift;
     return Py_BuildValue(
-        "KKKKKKK",
-        (unsigned long long) uv.npages << uv.pageshift,  // total
-        (unsigned long long) uv.free << uv.pageshift,  // free
-        (unsigned long long) uv.active << uv.pageshift,  // active
-        (unsigned long long) uv.inactive << uv.pageshift,  // inactive
-        (unsigned long long) uv.wired << uv.pageshift,  // wired
-        (unsigned long long) (uv.filepages + uv.execpages) * pagesize,  // cached
-        available << uv.pageshift  // available
+        "LLLLLL",
+        (long long) uv.npages << uv.pageshift,  // total
+        (long long) uv.free << uv.pageshift,  // free
+        (long long) uv.active << uv.pageshift,  // active
+        (long long) uv.inactive << uv.pageshift,  // inactive
+        (long long) uv.wired << uv.pageshift,  // wired
+        cached  // cached
     );
 }