Separate the module 'waitpid' from the module 'sys_wait'.

Separate the module 'waitpid' from the module 'sys_wait'.

* lib/sys_wait.in.h (waitpid): Declare only if the 'waitpid' module is
present.
* m4/sys_wait_h.m4 (gl_SYS_WAIT_MODULE_INDICATOR): Invoke
gl_MODULE_INDICATOR_FOR_TESTS.
(gl_SYS_WAIT_H_DEFAULTS): Initialize GNULIB_WAITPID.
* modules/sys_wait (Depends-on): Remove waitpid.
(Makefile.am): Substitute GNULIB_WAITPID.
* modules/waitpid (configure.ac): Invoke gl_SYS_WAIT_MODULE_INDICATOR.
* tests/test-sys_wait-c++.cc (GNULIB_NAMESPACE::waitpid): Check the
signature only if the 'waitpid' module is present.
* doc/posix-functions/waitpid.texi: Mention the 'waitpid' module.
* NEWS: Mention the change.
* modules/grantpt (Depends-on): Add waitpid.
* modules/wait-process (Depends-on): Likewise.