posix: fix glob bugs with long login names

Current glob implementation allows unlimited user name for home
directory construction on GLOB_TILDE case.  To accomplish it glob
either construct a name on stack if size are small enough (based
on current alloca_used) or in heap otherwise.

This patch simplifies storage allocation by using the same scratch
buffer for both get_rlogin_r and getpwnam_r.

This also syncs with gnulib commit 064df0b (glob: fix bugs with long
login names).

Checked on x86_64-linux-gnu and on a build using build-many-glibcs.py
for all major architectures.

	* posix/glob.c (GET_LOGIN_NAME_MAX): Remove.
	(glob): Use the same scratch buffer for both getlogin_r and
	getpwnam_r.  Don’t require preallocation of the login name.  This
	simplifies storage allocation, and corrects the handling of
	long login names.