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/network/networkd-conf.c

@@ -14,7 +14,7 @@ int manager_parse_config_file(Manager *m) {
 
         assert(m);
 
-        r = config_parse_config_file("networkd.conf",
+        r = config_parse_config_file("systemd/networkd.conf",
                                      "Network\0"
                                      "DHCPv4\0"
                                      "DHCPv6\0"