journald: move uid_for_system_journal() to uid-alloc-range.h

Let's move this helper call from journald specific code to src/basic/,
so that we can use it from sd-journal.

While we are at it, slightly extend it to also cover container uids,
which are also routed to the system journal now.

This places the call in uid-alloc-range.[ch] which contains similar
functions that match UID ranges for specific purposes.

src/basic/uid-alloc-range.c

@@ -122,3 +122,10 @@ bool gid_is_system(gid_t gid) {
 
         return gid <= defs->system_gid_max;
 }
+
+bool uid_for_system_journal(uid_t uid) {
+
+        /* Returns true if the specified UID shall get its data stored in the system journal. */
+
+        return uid_is_system(uid) || uid_is_dynamic(uid) || uid == UID_NOBODY || uid_is_container(uid);
+}