lib/util: Move next_token* functions to util_str.c

lib/util: Move next_token* functions to util_str.c

This is part of an attempt to untangle charset from util.c.

Most of this was moved from util_str.c in commit
55903e6f9120f1ec58a8554813229975c3028a09.  next_token() was moved from
charset to util.c in commit 4be643d4ce33d5ce2bf9deacc3f6d0fde90cf626
so that it would be alongside next_token_talloc().

Now that util_str.c is in shared lib/util it is no longer necessary to
have these in util.c.

Signed-off-by: Martin Schwenke <martin@meltin.net>
Reviewed-by: Volker Lendecke <vl@samba.org>