Add source file strlist.c

This is a copy from current GnuPG master. The license has been
chnaged from (LGPL-3.0-or-later OR GPL-2.0-or-later) to
LGPL-2.1-or-later: Almost everything was written by me or by employees
of g10 Code GmbH. The only exception is strlist_pop

2002-06-14  David Shaw  <dshaw@jabberwocky.com>

* strgutil.c (pop_strlist): New function to pop the head off of a
strlist.

from gnupg-1.4/util/strgutil.c. However the function is to obvious to
inhibit the license change. It needs to be changes anyway with
followup commits.

• Signed-off-by: Werner Koch <wk@gnupg.org>