nanosleep: work around cygwin bug

nanosleep: work around cygwin bug

Cygwin 1.5.x mistakenly failed with EINVAL for a duration longer
than 49.7 days (2**32 milliseconds).  Meanwhile, the existing
code for HAVE_BUG_BIG_NANOSLEEP would infloop, instead of return
failure, for invalid arguments.

* lib/nanosleep.c (rpl_nanosleep) [HAVE_BUG_BIG_NANOSLEEP]:
Fix logic bug when nanosleep fails.  Work around cygwin 1.5.x
bug.
(getnow): Delete, not needed.
* m4/nanosleep.m4 (gl_FUNC_NANOSLEEP): No longer require
LIB_CLOCK_GETTIME.
* modules/nanosleep (Depends-on): Add intprops and verify.  Drop
clock-time, gettime.
* doc/posix-functions/nanosleep.texi (nanosleep): Document the
bug.
* modules/nanosleep-tests: New test.
* tests/test-nanosleep.c: New file.

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