userns: Convert audit to work with user namespaces enabled

- Explicitly format uids gids in audit messges in the initial user
  namespace. This is safe because auditd is restrected to be in
  the initial user namespace.

- Convert audit_sig_uid into a kuid_t.

- Enable building the audit code and user namespaces at the same time.

The net result is that the audit subsystem now uses kuid_t and kgid_t whenever
possible making it almost impossible to confuse a raw uid_t with a kuid_t
preventing bugs.

Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Eric Paris <eparis@redhat.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>

kernel/audit.h

@@ -146,7 +146,7 @@ extern void audit_kill_trees(struct list_head *);
 extern char *audit_unpack_string(void **, size_t *, size_t);
 
 extern pid_t audit_sig_pid;
-extern uid_t audit_sig_uid;
+extern kuid_t audit_sig_uid;
 extern u32 audit_sig_sid;
 
 #ifdef CONFIG_AUDITSYSCALL