sleep: work around cygwin bug

sleep: work around cygwin bug

On cygwin 1.5.x, sleep amounts larger than 49.7 days (2**32
milliseconds) failed instantly, but with a garbage return
value from uninitialized memory.

* lib/sleep.c (rpl_sleep): Work around the bug.
* m4/sleep.m4 (gl_FUNC_SLEEP): Detect the bug.
(gl_PREREQ_SLEEP): Delete unused macro.
* modules/sleep (Depends-on): Add verify.
* m4/unistd_h.m4 (gl_UNISTD_H_DEFAULTS): Add default.
* modules/unistd (Makefile.am): Substitute witness.
* lib/unistd.in.h (sleep): Update prototype.
* doc/posix-functions/sleep.texi (sleep): Document the bug.
* tests/test-sleep.c (main) [HAVE_DECL_ALARM]: Test it.
* modules/sleep-tests (Depends-on): Check for alarm.

Signed-off-by: Eric Blake <ebb9@byu.net>