lib/chkname.c: Update regex for valid names

lib/chkname.c: Update regex for valid names

The maximum length of 32 wasn't being enforced in the code, and POSIX
doesn't specify that maximum length either, so it seems it was an
arbitrary limit of the past that doesn't exist any more.  Use a regex
that has no length limit.

Closes: <https://github.com/shadow-maint/shadow/issues/836>
Link: <https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap09.html>
Cc: Iker Pedrosa <ipedrosa@redhat.com>
Cc: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Alejandro Colomar <alx@kernel.org>