conf: replace config_parse_many_nulstr() with config_parse_config_file()

All daemons use a similar scheme to read their main config files and theirs
drop-ins. The main config files are always stored in /etc/systemd directory and
it's easy enough to construct the name of the drop-in directories based on the
name of the main config file.

Hence the new helper does that internally, which allows to reduce and simplify
the args passed previously to config_parse_many_nulstr().

Besides the overall code simplification it results:

  16 files changed, 87 insertions(+), 159 deletions(-)

it allows to identify clearly the locations in the code where configuration
files are parsed.

1 files changed, 3 insertions, 8 deletions

diff --git a/src/resolve/resolved-conf.c b/src/resolve/resolved-conf.c
index d6929984e9..70a6f99450 100644
--- a/src/resolve/resolved-conf.c
+++ b/src/resolve/resolved-conf.c
@@ -563,14 +563,9 @@ int manager_parse_config_file(Manager *m) {
 
         assert(m);
 
-        r = config_parse_many_nulstr(
-                        PKGSYSCONFDIR "/resolved.conf",
-                        CONF_PATHS_NULSTR("systemd/resolved.conf.d"),
-                        "Resolve\0",
-                        config_item_perf_lookup, resolved_gperf_lookup,
-                        CONFIG_PARSE_WARN,
-                        m,
-                        NULL);
+        r = config_parse_config_file("resolved.conf", "Resolve\0",
+                                     config_item_perf_lookup, resolved_gperf_lookup,
+                                     CONFIG_PARSE_WARN, m);
         if (r < 0)
                 return r;