Add support for the x-systemd.onfailure fstab option in fstab-generator.
This allows generated mount units to include the systemd OnFailure=
dependency specified through /etc/fstab.
x-systemd.onfailure= is handled by generalizing the existing
write_dependency() helper (already used for After=/Before=/Requires=/
Wants=) to take an optional unit-name suffix. When no suffix is given,
unit names are mangled without forcing a ".mount" suffix, since
OnFailure= targets are not necessarily mount units. write_on_failure()
is now a thin wrapper around write_dependency().
Example:
/dev/sda1 /data ext4 defaults,x-systemd.onfailure=emergency.target 0 2
generates:
OnFailure=emergency.target
This makes it possible to attach recovery targets or services to mount
failures directly from fstab configuration.
Add documentation and generator tests.