sd-event: split out enable and disable codepaths from sd_event_source_set_enabled()

sd-event: split out enable and disable codepaths from sd_event_source_set_enabled()

So far half of sd_event_source_set_enabled() was doing enabling, the
other half was doing disabling. Let's split that into two separate
calls.

(This also adds a new shortcut to sd_event_source_set_enabled(): if the
caller toggles between "ON" and "ONESHOT" we'll now shortcut this, since
the event source is already enabled in that case and shall remain
enabled.)

This heavily borrows and is inspired from Michal Sekletár's #17284
refactoring.