lib: Add "strv" string handling routines

lib: Add "strv" string handling routines

This is a little set of routines designed after the glibc argz
routines. It is supposed to eventually replace our inefficient string_list
routines. A talloc blob is an array of strings separated by the \0
character. See argz(3) on a Linux system for the ideas where this came
from. Based on talloc strv is simpler because talloc knows about the
size of the blob, so we don't have to explicitly maintain it.

Signed-off-by: Volker Lendecke <vl@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>