getlogin_r: Work around portability problem on OSF/1.

getlogin_r: Work around portability problem on OSF/1.

* m4/getlogin_r.m4 (gl_FUNC_GETLOGIN_R): Detect the OSF/1 problem.
* lib/unistd.in.h (getlogin_r): Replace if REPLACE_GETLOGIN_R is set.
* lib/getlogin_r.c (getlogin_r): When getlogin_r exists, invoke it and
test for a truncated result.
* m4/unistd_h.m4 (gl_UNISTD_H_DEFAULTS): Initialize REPLACE_GETLOGIN_R.
* modules/unistd (Makefile.am): Substitute REPLACE_GETLOGIN_R.
* modules/getlogin_r (Depends-on): Add memchr.
* doc/posix-functions/getlogin_r.texi: Mention the OSF/1 problem.