Fix strerror on Interix.

Fix strerror on Interix.

* lib/string_.h (strerror): Declare replacement.
* doc/functions/strerror.texi (strerror): Document the Interix
shortcoming.
* modules/string (Makefile.am): Support new hooks.
* m4/string_h.m4 (gl_HEADER_STRING_H_DEFAULTS): Add new hooks.
* m4/strerror.m4 (gl_FUNC_STRERROR): Defer to
gl_FUNC_STRERROR_SEPARATE.
(gl_FUNC_STRERROR_SEPARATE): Check for Interix bug.
* lib/strerror.c (rpl_strerror): Provide replacement.
* modules/strerror (Depends-on): Add string.
(configure.ac): Detect use of module.
* tests/test-strerror.c: New file.
* modules/strerror-tests: New test module.
* modules/argp (Depends-on): Add strerror.
* modules/error (Depends-on): Likewise.
Reported by Martin Koeppe.

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