systemd instancing support & rpm build improvements

The major things this does are adding systemd support to the rpm .spec
file, and adding systemd instancing support. This means that it is
possible to run multiple memcached instances without having to do
any additional configuration or hack on init scripts.

To use:
    systemctl start memcached@11211 memcached@11311 memcached@11411

sysconfig files at /etc/sysconfig/memcached.<port> will be read as
appropriate, to allow differing configurations per-port. Defaults
will be read from /etc/sysconfig/memcached before the port-specific
settings are read.

You can also still start memcached the standard way just by doing
"systemctl start memcached". This will read /etc/sysconfig/memcached
and nothing else.

The "enhanced security" lines in the systemd unit file will be commented
out on systems where we know systemd isn't knew enough (fedora < 26 and
Redhat/CentOS 7), and enabled on other systems.

There are two versions of the .service file included, one for standard
memcached invocations and one for instanced invocations. The two are
very similar, but not identical. Ideally, we'd only have one version
in the source tree and we'd massage it with sed or somesuch during the
rpm build, but couldn't think of a super clean way to do that, so erred
on the side of simplicity.

A decent amount of spec file work was needed to enable this functionality.
In the process, I also cleaned up several additional aspects of the spec
file (like using %{name} in places where it was appropriate). I also
commented out the automatic restart in the %postun section, for two main
reasons:
    1. The try-restart for instanced memcached will produce an error if
    instanced memcached isn't in use, which is probably quite confusing to
    people who aren't using that functionality and are just trying to update
    their package. (There's workarounds for this, but I try to keep pre/post
    scripts as simple as humanly possible)

    2. Automatic restarts on updates means the cache gets flushed, which
    means you can no longer safely use large-scale management tools (like
    puppet or chef) to roll out new versions, at least not without a lot
    of planning first. Not automatically dumping someone's caches feels
    safer, here.

1 files changed, 10 insertions, 0 deletions

diff --git a/scripts/memcached.sysconfig b/scripts/memcached.sysconfig
new file mode 100644
index 0000000..bd46483
--- /dev/null
+++ b/scripts/memcached.sysconfig
@@ -0,0 +1,10 @@
+# These defaults will be used by every memcached instance, unless overridden
+# by values in /etc/sysconfig/memcached.<port>
+USER="nobody"
+MAXCONN="1024"
+CACHESIZE="64"
+OPTIONS=""
+
+# The PORT variable will only be used by memcached.service, not by
+# memcached@xxxxx services, which will use the xxxxx
+PORT="11211"