core: add IgnoreOnSoftReboot= unit option

As it says on the tin, configures the unit to survive a soft reboot.
Currently all the following options have to be set by hand:

Conflicts=reboot.target kexec.target poweroff.target halt.target
Before=reboot.target kexec.target poweroff.target halt.target
After=sysinit.target basic.target
DefaultDependencies=no
IgnoreOnIsolate=yes

This is not very user friendly. If new default dependencies are added,
or new shutdown/reboot types, they also have to be added manually.

The new option is much simpler, easy to find, and does the right thing
by default.

src/shared/bus-unit-util.c

@@ -2521,6 +2521,7 @@ static int bus_append_unit_property(sd_bus_message *m, const char *field, const
                               "RefuseManualStop",
                               "AllowIsolate",
                               "IgnoreOnIsolate",
+                              "IgnoreOnSoftReboot",
                               "DefaultDependencies"))
                 return bus_append_parse_boolean(m, field, eq);