usleep: new module

usleep: new module

mingw usleep(1000000) failed with EINVAL, as allowed by POSIX,
but contrary to GNU usage.  Rather than implement an accurate
usleep based on select or nanosleep, both of which drag in
dependencies on external libraries, this version intentionally
takes the ceiling in seconds if usleep() is missing.

* modules/usleep: New file.
* m4/usleep.m4 (gl_FUNC_USLEEP): Likewise.
* lib/usleep.c (usleep): Likewise.
* m4/unistd_h.m4 (gl_UNISTD_H_DEFAULTS): Add defaults.
* modules/unistd (Makefile.am): Substitute witnesses.
* lib/unistd.in.h (usleep): Add declaration.
* doc/pastposix-functions/usleep.texi (usleep): Document this.
* MODULES.html.sh (Date and time): Likewise.
* modules/usleep-tests (Depends-on): New test.
* tests/test-usleep.c: New file.

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