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.

test/test-fstab-generator/test-18-options.fstab.expected.sysroot/local-fs.target.requires/mnt-onfailure.mount

@@ -0,0 +1 @@
+../mnt-onfailure.mount

test/test-fstab-generator/test-18-options.fstab.expected.sysroot/local-fs.target.requires/mnt-onfailuremulti.mount

@@ -0,0 +1 @@
+../mnt-onfailuremulti.mount

test/test-fstab-generator/test-18-options.fstab.expected.sysroot/mnt-onfailure.mount

@@ -0,0 +1,13 @@
+# Automatically generated by systemd-fstab-generator
+
+[Unit]
+Documentation=man:fstab(5) man:systemd-fstab-generator(8)
+SourcePath=/etc/fstab
+OnFailure=foo.service
+Before=local-fs.target
+After=blockdev@dev-sdx18.target
+
+[Mount]
+What=/dev/sdx18
+Where=/mnt/onfailure
+Options=x-systemd.onfailure=foo.service

test/test-fstab-generator/test-18-options.fstab.expected.sysroot/mnt-onfailuremulti.mount

@@ -0,0 +1,13 @@
+# Automatically generated by systemd-fstab-generator
+
+[Unit]
+Documentation=man:fstab(5) man:systemd-fstab-generator(8)
+SourcePath=/etc/fstab
+OnFailure=foo.service bar.service
+Before=local-fs.target
+After=blockdev@dev-sdx19.target
+
+[Mount]
+What=/dev/sdx19
+Where=/mnt/onfailuremulti
+Options=x-systemd.onfailure=foo.service,x-systemd.onfailure=bar.service

test/test-fstab-generator/test-18-options.fstab.expected/local-fs.target.requires/mnt-onfailure.mount

@@ -0,0 +1 @@
+../mnt-onfailure.mount

test/test-fstab-generator/test-18-options.fstab.expected/local-fs.target.requires/mnt-onfailuremulti.mount

@@ -0,0 +1 @@
+../mnt-onfailuremulti.mount

test/test-fstab-generator/test-18-options.fstab.expected/mnt-onfailure.mount

@@ -0,0 +1,13 @@
+# Automatically generated by systemd-fstab-generator
+
+[Unit]
+Documentation=man:fstab(5) man:systemd-fstab-generator(8)
+SourcePath=/etc/fstab
+OnFailure=foo.service
+Before=local-fs.target
+After=blockdev@dev-sdx18.target
+
+[Mount]
+What=/dev/sdx18
+Where=/mnt/onfailure
+Options=x-systemd.onfailure=foo.service

test/test-fstab-generator/test-18-options.fstab.expected/mnt-onfailuremulti.mount

@@ -0,0 +1,13 @@
+# Automatically generated by systemd-fstab-generator
+
+[Unit]
+Documentation=man:fstab(5) man:systemd-fstab-generator(8)
+SourcePath=/etc/fstab
+OnFailure=foo.service bar.service
+Before=local-fs.target
+After=blockdev@dev-sdx19.target
+
+[Mount]
+What=/dev/sdx19
+Where=/mnt/onfailuremulti
+Options=x-systemd.onfailure=foo.service,x-systemd.onfailure=bar.service

test/test-fstab-generator/test-18-options.fstab.input

@@ -15,3 +15,5 @@
 /dev/sdx15 /mnt/noauto             auto noauto                                              0 0
 /dev/sdx16 /mnt/nofail             auto nofail                                              0 0
 /dev/sdx17 /mnt/wantedby-automount auto x-systemd.wanted-by=foo.service,x-systemd.automount 0 0
+/dev/sdx18 /mnt/onfailure          auto x-systemd.onfailure=foo.service                     0 0
+/dev/sdx19 /mnt/onfailuremulti     auto x-systemd.onfailure=foo.service,x-systemd.onfailure=bar.service 0 0