New module 'poll-h'.

New module 'poll-h'.

* lib/poll.in.h: Include c++defs.h and warn-on-use.h.
(poll): Use modern idiom.
* modules/poll-h: New file.
* modules/poll (Files): Remove lib/poll.in.h.
(Depends-on): Add poll-h.
(configure.ac): Invoke gl_POLL_MODULE_INDICATOR.
(Makefile.am): Move code for generation of poll.h to modules/poll-h.
* m4/poll_h.m4: New file.
* m4/poll.m4 (gl_FUNC_POLL): Require gl_POLL_H. Don't check for poll.h
here. Don't set POLL_H here. Instead, set HAVE_POLL and REPLACE_POLL
and invoke gl_REPLACE_POLL_H.
* lib/poll.c: Use common idiom.
* tests/test-poll.c: Likewise.
* doc/posix-headers/poll.texi: Mention the poll-h module.
Suggested by Eric Blake.