core: add WantsMountsFor=

This is the equivalent of RequiresMountsFor=, but adds Wants= instead
of Requires=. It will be useful for example for the autogenerated
systemd-cryptsetup units.

Fixes https://github.com/systemd/systemd/issues/11646

src/shared/bus-unit-util.c

@@ -2643,6 +2643,7 @@ static int bus_append_unit_property(sd_bus_message *m, const char *field, const
         if (unit_dependency_from_string(field) >= 0 ||
             STR_IN_SET(field, "Documentation",
                               "RequiresMountsFor",
+                              "WantsMountsFor",
                               "Markers"))
                 return bus_append_strv(m, field, eq, EXTRACT_UNQUOTE);