user-accounts: Fix definition of MAXNAMELEN

Currently we get MAXNAMELEN from glibc's LOGIN_NAME_MAX, if available,
and sysconf otherwise. But the maximum username length supported by
glibc and the kernel is actually way larger than the maximum length that
actually works in practice. On Linux, anything larger than 32 characters
is not going to fit into utmp, and will therefore be rejected by
useradd. Then gnome-control-center will spit out a confusing error
message dialog.

Let's spare our users from that.

useradd (in shadow-utils) gets its max name size from the following
magic:

 /* Maximum length of usernames */
 #ifdef HAVE_UTMPX_H
 # include <utmpx.h>
 # define USER_NAME_MAX_LENGTH (sizeof (((struct utmpx *)NULL)->ut_user))
 #else
 # include <utmp.h>
 # ifdef HAVE_STRUCT_UTMP_UT_USER
 #  define USER_NAME_MAX_LENGTH (sizeof (((struct utmp *)NULL)->ut_user))
 # else
 #  ifdef HAVE_STRUCT_UTMP_UT_NAME
 #   define USER_NAME_MAX_LENGTH (sizeof (((struct utmp *)NULL)->ut_name))
 #  else
 #   define USER_NAME_MAX_LENGTH 32
 #  endif
 # endif
 #endif

It's more work than necessary. utmpx is standardized by POSIX (it's
actually an XSI extension), whereas utmp is not, so let's just use
utmpx. This ought to work on at least FreeBSD as well. And if any free
operating systems that care about GNOME don't have utmpx yet, no doubt
they'll send patches.

https://bugzilla.gnome.org/show_bug.cgi?id=724193