Support for MSVC compiler: Ensure mode_t gets defined.

Support for MSVC compiler: Ensure mode_t gets defined.

* m4/fcntl_h.m4 (gl_FCNTL_H): Require AC_TYPE_MODE_T.
* m4/spawn_h.m4 (gl_SPAWN_H): Likewise.
* m4/sys_stat_h.m4 (gl_HEADER_SYS_STAT_H): Likewise.
* m4/sys_types_h.m4 (gl_SYS_TYPES_H): Likewise.
* tests/test-fcntl-h.c: Check that mode_t is defined.
* tests/test-sys_stat.c: Likewise.
* tests/test-sys_types.c: Likewise.
* doc/posix-headers/fcntl.texi: Mention the missing mode_t problem.
* doc/posix-headers/sys_stat.texi: Likewise.
* doc/posix-headers/sys_types.texi: Likewise.