logind: add Halt() and CanHalt() APIs

This adds new method calls Halt() and CanHalt() to the logind bus APIs.
They aren't overly useful (as the whole concept of halting isn't really
too useful), however they clean up one major asymmetry: currently, using
the "shutdown" legacy commands it is possibly to enqueue a "halt"
operation through logind, while logind officially doesn't actually
support this. Moreover, the path through "shutdown" currently ultimately
fails, since the referenced "halt" action isn't actually defined in
PolicyKit.

Finally, the current logic results in an unexpected asymmetry in
systemctl: "systemctl poweroff", "systemctl reboot" are currently
asynchronous (due to the logind involvement) while "systemctl halt"
isnt. Let's clean this up, and make all three APIs implemented by
logind natively, and all three hence asynchronous in "systemctl".

Moreover, let's add the missing PK action.

Fixes: #6957

src/systemctl/systemctl.c

@@ -3267,14 +3267,19 @@ static int logind_reboot(enum action a) {
 
         switch (a) {
 
+        case ACTION_POWEROFF:
+                method = "PowerOff";
+                description = "power off system";
+                break;
+
         case ACTION_REBOOT:
                 method = "Reboot";
                 description = "reboot system";
                 break;
 
-        case ACTION_POWEROFF:
-                method = "PowerOff";
-                description = "power off system";
+        case ACTION_HALT:
+                method = "Halt";
+                description = "halt system";
                 break;
 
         case ACTION_SUSPEND:
@@ -3568,6 +3573,7 @@ static int start_special(int argc, char *argv[], void *userdata) {
                 if (IN_SET(a,
                            ACTION_POWEROFF,
                            ACTION_REBOOT,
+                           ACTION_HALT,
                            ACTION_SUSPEND,
                            ACTION_HIBERNATE,
                            ACTION_HYBRID_SLEEP)) {
@@ -8503,7 +8509,7 @@ static int halt_main(void) {
                 /* Try logind if we are a normal user and no special
                  * mode applies. Maybe PolicyKit allows us to shutdown
                  * the machine. */
-                if (IN_SET(arg_action, ACTION_POWEROFF, ACTION_REBOOT)) {
+                if (IN_SET(arg_action, ACTION_POWEROFF, ACTION_REBOOT, ACTION_HALT)) {
                         r = logind_reboot(arg_action);
                         if (r >= 0)
                                 return r;