Move x-systemd-device.timeout handling from core to fstab-generator

Move x-systemd-device.timeout handling from core to fstab-generator

Instead of adjusting job timeouts in the core, let fstab-generator
write out a dropin snippet with the appropriate JobTimeout.
x-systemd-device.timeout option is removed from Options= line
in the generated unit.

The functions to write dropins are moved from core/unit.c to
shared/dropin.c, to make them available outside of core.

generator.c is moved to libsystemd-label, because it now uses
functions defined in dropin.c, which are in libsystemd-label.