check_dig: query transport selection support

Add command line options -4 and -6 (--use-ipv4 and
--use-ipv6) to check_dig.

Effectively this adds no new functionality, just a
shortcut for:
  check_dig -A "-4"
  check_dig -A "-6"

Needed for using check_dig with the check_v46 wrapper
(see e.g.
https://gitorious.org/nagios-monitoring-tools/nagios-monitoring-tools/blobs/master/check_v46)
which requires -4/-6 command line option support.