Currently this driver tracks user space clients it should send signals to.  In
the presenct of file descriptor passing this is appears susceptible to
confusion from pid wrap around issues.

Replacing this with a struct pid prevents us from getting confused, and
prepares for a pid namespace implementation.
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
Cc: David Woodhouse <dwmw2@infradead.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>