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.

src/basic/constants.h

@@ -67,10 +67,8 @@
 #  define _CONF_PATHS_SPLIT_USR(n)
 #endif
 
-/* Return a nulstr for a standard cascade of configuration paths,
- * suitable to pass to conf_files_list_nulstr() or config_parse_many_nulstr()
- * to implement drop-in directories for extending configuration
- * files. */
+/* Return a nulstr for a standard cascade of configuration paths, suitable to pass to
+ * conf_files_list_nulstr() to implement drop-in directories for extending configuration files. */
 #define CONF_PATHS_NULSTR(n)                    \
         "/etc/" n "\0"                          \
         "/run/" n "\0"                          \