shared/conf-parser: collapse pkgdir and conf_file args into one

This essentially reverts 5656cdfeea. I find it
much easier to understand what is going on when the
path-relative-to-the-search-path is passed in full, instead of being constructed
from two parts, with one of the parts being implicit in some places.

Also, we call 'systemd-analyze cat-config <path>' with <path> with the same
meaning, so this makes the internal and external APIs more consistent.

src/resolve/resolved-conf.c

@@ -570,7 +570,7 @@ int manager_parse_config_file(Manager *m) {
 
         assert(m);
 
-        r = config_parse_config_file("resolved.conf", "Resolve\0",
+        r = config_parse_config_file("systemd/resolved.conf", "Resolve\0",
                                      config_item_perf_lookup, resolved_gperf_lookup,
                                      CONFIG_PARSE_WARN, m);
         if (r < 0)