[PATCH] uml: umid tidying

Add an error message when two umids are put on the command line.

umid.h is kind of pointless since it only declares one thing, and that
is already declared in os.h.

Commented the lack of locking of some data in os-Linux/umid.h.

Signed-off-by: Jeff Dike <jdike@addtoit.com>
Cc: Paolo 'Blaisorblade' Giarrusso <blaisorblade@yahoo.it>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>