s3: Add wbinfo --dc-info

s3: Add wbinfo --dc-info

wbinfo --dc-info prints the current DC name and IP address. This helps
diagnosing problems that might happen when a later wbinfo --ping-dc fails.

This patch started out by using the SAF and NBT cache entires, but those are
relatively short-lived. So I decided to invent a new gencache entry with a very
long timeout. We need to go via the gencache because when for some reason a
winbind child process is stuck, we can't query it for the current DC it's
connected to. This must eventually go away again when we have a fully async
winbind.

Autobuild-User: Volker Lendecke <vlendec@samba.org>
Autobuild-Date: Wed Jan 19 08:40:28 CET 2011 on sn-devel-104