fstab-generator: support for x-systemd.onfailure

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.

man/systemd.mount.xml

@@ -284,6 +284,25 @@
         <xi:include href="version-info.xml" xpointer="v220"/></listitem>
        </varlistentry>
 
+      <varlistentry>
+        <term><option>x-systemd.onfailure=</option></term>
+
+        <listitem><para>In the created mount unit, configures an
+        <varname>OnFailure=</varname> dependency. The argument is a unit name, see
+        <option>x-systemd.requires=</option> above. This option may be specified more than
+        once. See <varname>OnFailure=</varname> in
+        <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>
+        for details.</para>
+
+        <para>Note that this option always applies to the created mount
+        unit only, regardless of whether <option>x-systemd.automount</option>
+        has been specified.</para>
+
+        <para>Note that this option is silently ignored on swap entries.</para>
+
+        <xi:include href="version-info.xml" xpointer="v262"/></listitem>
+      </varlistentry>
+
       <varlistentry>
         <term><option>x-systemd.graceful-option=</option></term>