samba-tool gpo: improve UNC parsing

samba-tool gpo: improve UNC parsing

The "UNC doesn't start with \\\\ or //" message was unreachable due to
a logic error, and an UNC starting with \\ would have been split on
/ if there were enough /s in the string.

The unreachable exception was first noticed by Gerhard Lausser in a
github pull request (https://github.com/samba-team/samba/pull/123),
but that patch no longer applies with this more thorough rewrite.

Signed-off-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Jeremy Allison <jra@samba.org>