smb: implement kdirnotify support

to track file changes/add/removals for smb:// uris

since smbclient isn't properly thread safe right now, we use separate
processes per URI to notify on changes

a kded module listens to kdirnotify dir enter events and spawns a
notifier subprocess. the notifier then attempts to open a notify loop
with the remote. authentication ought to be handled via the kiod auth
cache, that is to say: when cached auth isn't available or not valid the
notifier will simply not work at all. there's no interactive auth
queries as they'd lack context from a user POV