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/core/unit.c

@@ -3291,19 +3291,46 @@ int unit_add_dependency(
 }
 
 int unit_add_two_dependencies(Unit *u, UnitDependency d, UnitDependency e, Unit *other, bool add_reference, UnitDependencyMask mask) {
-        int r, s;
+        int r = 0, s = 0;
+
+        assert(u);
+        assert(d >= 0 || e >= 0);
+
+        if (d >= 0) {
+                r = unit_add_dependency(u, d, other, add_reference, mask);
+                if (r < 0)
+                        return r;
+        }
+
+        if (e >= 0) {
+                s = unit_add_dependency(u, e, other, add_reference, mask);
+                if (s < 0)
+                        return s;
+        }
+
+        return r > 0 || s > 0;
+}
+
+int unit_add_dependencies_on_real_shutdown_targets(Unit *u) {
+        int r;
 
         assert(u);
 
-        r = unit_add_dependency(u, d, other, add_reference, mask);
-        if (r < 0)
-                return r;
+        STRV_FOREACH(target, STRV_MAKE(SPECIAL_REBOOT_TARGET,
+                                SPECIAL_KEXEC_TARGET,
+                                SPECIAL_HALT_TARGET,
+                                SPECIAL_POWEROFF_TARGET)) {
+                r = unit_add_two_dependencies_by_name(u,
+                                UNIT_BEFORE,
+                                UNIT_CONFLICTS,
+                                *target,
+                                /* add_reference= */ true,
+                                UNIT_DEPENDENCY_DEFAULT);
+                if (r < 0)
+                        return r;
+        }
 
-        s = unit_add_dependency(u, e, other, add_reference, mask);
-        if (s < 0)
-                return s;
-
-        return r > 0 || s > 0;
+        return 0;
 }
 
 static int resolve_template(Unit *u, const char *name, char **buf, const char **ret) {