journald: move uid_for_system_journal() to uid-alloc-range.h

journald: move uid_for_system_journal() to uid-alloc-range.h

Let's move this helper call from journald specific code to src/basic/,
so that we can use it from sd-journal.

While we are at it, slightly extend it to also cover container uids,
which are also routed to the system journal now.

This places the call in uid-alloc-range.[ch] which contains similar
functions that match UID ranges for specific purposes.