unistd_h: make it easier to avoid sys_types_h

unistd_h: make it easier to avoid sys_types_h

This is useful for Emacs, which has its own method of porting to
Windows, and which therefore does not need the sys_types_h module.
* m4/off_t.m4: New file, defining gl_TYPE_OFF_T, which contains
code moved here from gl_SYS_TYPES_H.
* m4/sys_types_h.m4 (gl_SYS_TYPES_H): Require it instead of
using the code directly.
* m4/unistd_h.m4 (gl_UNISTD_H): Require gl_TYPE_OFF_T, not
gl_SYS_TYPES_H.
* modules/sys_types (Files):
* modules/unistd (Files): Add m4/off_t.m4.