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, 8 insertions, 11 deletions

diff --git a/src/network/networkd-conf.c b/src/network/networkd-conf.c
index f73b2d43c7..063732a3b4 100644
--- a/src/network/networkd-conf.c
+++ b/src/network/networkd-conf.c
@@ -14,17 +14,14 @@ int manager_parse_config_file(Manager *m) {
 
         assert(m);
 
-        r = config_parse_many_nulstr(
-                        PKGSYSCONFDIR "/networkd.conf",
-                        CONF_PATHS_NULSTR("systemd/networkd.conf.d"),
-                        "Network\0"
-                        "DHCPv4\0"
-                        "DHCPv6\0"
-                        "DHCP\0",
-                        config_item_perf_lookup, networkd_gperf_lookup,
-                        CONFIG_PARSE_WARN,
-                        m,
-                        NULL);
+        r = config_parse_config_file("networkd.conf",
+                                     "Network\0"
+                                     "DHCPv4\0"
+                                     "DHCPv6\0"
+                                     "DHCP\0",
+                                     config_item_perf_lookup, networkd_gperf_lookup,
+                                     CONFIG_PARSE_WARN,
+                                     m);
         if (r < 0)
                 return r;