shared/sleep-config: add switches to kill specific sleep modes

shared/sleep-config: add switches to kill specific sleep modes

/etc/systemd/sleep.conf gains four new switches:
AllowSuspend=, AllowHibernation=, AllowSuspendThenHibernate=, AllowHybridSleep=.

Disabling specific modes was already possible by masking suspend.target,
hibernate.target, suspend-then-hibernate.target, or hybrid-sleep.target.
But this is not convenient for distributions, which want to set some defaults
based on what they want to support. Having those available as configuration
makes it easy to put a config file in /usr/lib/systemd/sleep.conf.d/ that
overrides the defaults and gives instructions how to undo that override.