Revert "lib/util: Expose strv_addn() for adding string with specified length"

Revert "lib/util: Expose strv_addn() for adding string with specified length"

This reverts commit 0c61dd15137b4603bd47b3d6ae18ded6bd18ffae.

The intention of strv_addn() is to be able to add some number of
characters from an existing string.  This implementation carelessly
assumes that the old _strv_append() added the trailing NUL to form a
valid strv.  That's not true.

New implementation to follow.

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