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/coredump/coredump.c b/src/coredump/coredump.c
index d5d1f49d08..e715fd232b 100644
--- a/src/coredump/coredump.c
+++ b/src/coredump/coredump.c
@@ -171,14 +171,9 @@ static int parse_config(void) {
                 {}
         };
 
-        return config_parse_many_nulstr(
-                        PKGSYSCONFDIR "/coredump.conf",
-                        CONF_PATHS_NULSTR("systemd/coredump.conf.d"),
-                        "Coredump\0",
-                        config_item_table_lookup, items,
-                        CONFIG_PARSE_WARN,
-                        NULL,
-                        NULL);
+        return config_parse_config_file("coredump.conf", "Coredump\0",
+                                        config_item_table_lookup, items,
+                                        CONFIG_PARSE_WARN, NULL);
 }
 
 static uint64_t storage_size_max(void) {