s3:net: add net_update_dns_ext() that accepts a list of addresses as parameter (bug...

s3:net: add net_update_dns_ext() that accepts a list of addresses as parameter (bug# 7871)

This generalized form of net_update_dns() will be used to
add support for specifying a list of addresses on the commandline
of "net ads dns register".

This prepares the "net ads dns register" part of the fix for bug #7871.

Signed-off-by: Stefan Metzmacher <metze@samba.org>
(cherry picked from commit 4d91f98b433e07922373bf4e3ba9668b7af71a00)
(cherry picked from commit 4b7775500b1055acf62decbc0fc8283b088da452)