sleep: Add debug feature to bypass hibernation memory checks. (#3064)

sleep: Add debug feature to bypass hibernation memory checks. (#3064)

This new feature bypasses checking if a swap partition is mounted
or if there is enough swap space available for hibernation to
succeed.

This can be useful when a system with a Solid State Disk (SSD)
has no normal swap partition or file configured, and a custom
systemd unit is used to mount a swap file just before hibernating
and unmount it just after resuming.

Signed-off-by: Vittorio Gambaletta <git-systemd@vittgam.net>