uml: current.h cleanup

Tidy current-related stuff.  There was a comment in current.h saying
that current_thread was obsolete, so this patch turns all instances of
current_thread into current_thread_info().  There's some simplifying
of the result in arch/um/sys-i386/signal.c.

current.h and thread_info also get style cleanups.

Signed-off-by: Jeff Dike <jdike@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>