tree-wide: use "polkit" to refer to PolicyKit/polkit

Back in 2012 the project was renamed, see the release notes for v 0.105
[https://cgit.freedesktop.org/polkit/tree/NEWS#n754]. Let's update our
documentation and comments to do the same. Referring to PolicyKit is confusing
to users because at the time the polkit api changed too, and we support the new
version. I updated NEWS too, since all the references to PolicyKit there were
added after the rename.

"PolicyKit" is unchanged in various URLs and method call names.

src/systemctl/systemctl.c

@@ -3178,7 +3178,7 @@ static int logind_set_wall_message(void) {
 #endif
 
 /* Ask systemd-logind, which might grant access to unprivileged users
- * through PolicyKit */
+ * through polkit */
 static int logind_reboot(enum action a) {
 #if ENABLE_LOGIND
         _cleanup_(sd_bus_error_free) sd_bus_error error = SD_BUS_ERROR_NULL;
@@ -8414,7 +8414,7 @@ static int halt_main(void) {
                 }
 
                 /* Try logind if we are a normal user and no special
-                 * mode applies. Maybe PolicyKit allows us to shutdown
+                 * mode applies. Maybe polkit allows us to shutdown
                  * the machine. */
                 if (IN_SET(arg_action, ACTION_POWEROFF, ACTION_REBOOT, ACTION_HALT)) {
                         r = logind_reboot(arg_action);