Kernel: Strongly typed user & group ID's

Prior to this change, both uid_t and gid_t were typedef'ed to `u32`.
This made it easy to use them interchangeably. Let's not allow that.

This patch adds UserID and GroupID using the AK::DistinctNumeric
mechanism we've already been employing for pid_t/ProcessID.

Kernel/FileSystem/FileDescription.h

@@ -123,7 +123,7 @@ public:
 
     off_t offset() const { return m_current_offset; }
 
-    KResult chown(uid_t, gid_t);
+    KResult chown(UserID, GroupID);
 
     FileBlockerSet& blocker_set();