common: Debug ctdb_addr_to_str() using new function ctdb_external_trace()

common: Debug ctdb_addr_to_str() using new function ctdb_external_trace()

We've seen this function report "Unknown family, 0" and then CTDB
disappeared without a trace.  If we can reproduce it then this might
help us to debug it.

The idea is that you do something like the following in /etc/sysconfig/ctdb:

  export CTDB_EXTERNAL_TRACE="/etc/ctdb/config/gcore_trace.sh"

When we hit this error than we call out to gcore to get a core file so
we can do forensics.  This might block CTDB for a few seconds.

Signed-off-by: Martin Schwenke <martin@meltin.net>