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, 16 insertions, 17 deletions

diff --git a/src/core/main.c b/src/core/main.c
index 3f63150f31..5f8f251e9a 100644
--- a/src/core/main.c
+++ b/src/core/main.c
@@ -704,33 +704,32 @@ static int parse_config_file(void) {
                 {}
         };
 
-        _cleanup_strv_free_ char **files = NULL, **dirs = NULL;
-        const char *suffix;
-        int r;
-
         if (arg_runtime_scope == RUNTIME_SCOPE_SYSTEM)
-                suffix = "system.conf.d";
+                (void) config_parse_config_file("system.conf",
+                                                "Manager\0",
+                                                config_item_table_lookup, items,
+                                                CONFIG_PARSE_WARN,
+                                                NULL);
         else {
+                _cleanup_strv_free_ char **files = NULL, **dirs = NULL;
+                int r;
+
                 assert(arg_runtime_scope == RUNTIME_SCOPE_USER);
 
                 r = manager_find_user_config_paths(&files, &dirs);
                 if (r < 0)
                         return log_error_errno(r, "Failed to determine config file paths: %m");
 
-                suffix = "user.conf.d";
+                (void) config_parse_many(
+                                (const char* const*) files,
+                                (const char* const*) dirs,
+                                "user.conf.d",
+                                "Manager\0",
+                                config_item_table_lookup, items,
+                                CONFIG_PARSE_WARN,
+                                NULL, NULL, NULL);
         }
 
-        (void) config_parse_many(
-                        (const char* const*) (files ?: STRV_MAKE(PKGSYSCONFDIR "/system.conf")),
-                        (const char* const*) (dirs ?: CONF_PATHS_STRV("systemd")),
-                        suffix,
-                        "Manager\0",
-                        config_item_table_lookup, items,
-                        CONFIG_PARSE_WARN,
-                        NULL,
-                        NULL,
-                        NULL);
-
         /* Traditionally "0" was used to turn off the default unit timeouts. Fix this up so that we use
          * USEC_INFINITY like everywhere else. */
         if (arg_default_timeout_start_usec <= 0)