getpass-gnu: Allow use as dependency from test modules.
* m4/getpass.m4 (gl_FUNC_GETPASS_GNU): Set
REPLACE_GETPASS_FOR_GETPASS_GNU instead of REPLACE_GETPASS.
* m4/unistd_h.m4 (gl_UNISTD_H_REQUIRE_DEFAULTS): Initialize the
getpass-gnu module indicator.
(gl_UNISTD_H_DEFAULTS): Initialize REPLACE_GETPASS_FOR_GETPASS_GNU.
* modules/unistd (Makefile.am): Substitute GNULIB_GETPASS_GNU,
REPLACE_GETPASS_FOR_GETPASS_GNU.
* modules/getpass-gnu (configure.ac): Test
REPLACE_GETPASS_FOR_GETPASS_GNU instead of REPLACE_GETPASS. Set a module
indicator.
* lib/unistd.in.h (getpass): Test REPLACE_GETPASS,
REPLACE_GETPASS_FOR_GETPASS_GNU, and the respective module indicators
instead of just REPLACE_GETPASS.