Swap sys/poll.h with poll.h.

Swap sys/poll.h with poll.h.

Similarly to (sys/)syslog.h, poll.h is the header standardized by
POSIX, but we had all of its contents in sys/, for historical reasons.
This patch exchanges the contents of the two headers, and adds
multiple-include guards to all of poll.h’s bits headers.

	* io/poll.h: Exchange contents with...
	* io/sys/poll.h: ...this file.  Adjust guard macros.

	* include/poll.h: Exchange contents with...
	* include/sys/poll.h: ...this file.  Adjust guard macros.

	* bits/poll.h, io/bits/poll2.h
	* sysdeps/unix/sysv/linux/bits/poll.h
	* sysdeps/unix/sysv/linux/m68k/bits/poll.h
	* sysdeps/unix/sysv/linux/mips/bits/poll.h
	* sysdeps/unix/sysv/linux/sparc/bits/poll.h:
        Allow inclusion by poll.h, not sys/poll.h.  Add multiple-
        include guards where not already present.

	* scripts/check-obsolete-constructs.py (HEADER_ALLOWED_INCLUDES):
        Update.