SIGIO-driven I/O with inotify queues

Add SIGIO-driven I/O for descriptors returned by inotify_init().  The thing
may be enabled by convenient fcntl (fd, F_SETFL, O_ASYNC) call.

Signed-off-by: Dmitry Antipov <antipov@dev.rtsoft.ru>
Cc: Robert Love <rlove@google.com>
Cc: John McCutchan <ttb@tentacle.dhs.org>
Cc: Michael Kerrisk <mtk-manpages@gmx.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>