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/oom/oomd.c

@@ -31,7 +31,7 @@ static int parse_config(void) {
                 {}
         };
 
-        return config_parse_config_file("oomd.conf", "OOM\0",
+        return config_parse_config_file("systemd/oomd.conf", "OOM\0",
                                         config_item_table_lookup, items,
                                         CONFIG_PARSE_WARN, NULL);
 }