readlink: fix cygwin 1.5.x bug with return type

readlink: fix cygwin 1.5.x bug with return type

On older systems, readlink returned int instead of ssize_t, making
the use of readlink via function pointer harder.

* m4/readlink.m4 (gl_FUNC_READLINK): Require correct signature.
* lib/unistd.in.h (readlink): Use ssize_t.
* lib/readlink.c (readlink): Likewise.
* m4/unistd_h.m4 (gl_UNISTD_H_DEFAULTS): Add witness.
* modules/unistd (Makefile.am): Substitute it.
* lib/unistd.in.h (readlink): Declare replacement.
* doc/posix-functions/readlink.texi (readlink): Document this.

Signed-off-by: Eric Blake <ebb9@byu.net>