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: