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/InodeFile.cpp

@@ -127,7 +127,7 @@ KResult InodeFile::truncate(u64 size)
     return KSuccess;
 }
 
-KResult InodeFile::chown(FileDescription& description, uid_t uid, gid_t gid)
+KResult InodeFile::chown(FileDescription& description, UserID uid, GroupID gid)
 {
     VERIFY(description.inode() == m_inode);
     VERIFY(description.custody());