BUG/MINOR: log: gracefully handle the "udp@" address format for log servers

BUG/MINOR: log: gracefully handle the "udp@" address format for log servers

Commit 3835c0dcb ("MEDIUM: udp: adds minimal proto udp support for
message listeners.") introduced a problematic side effect in log server
address parser: if "udp@", "udp4@" or "udp6@" prefixes a log server's
address, the adress is passed as-is to the log server with a non-existing
family and fails like this when trying to send:

  [ALERT] 259/195708 (3474) : socket() failed in logger #1: Address family not supported by protocol (errno=97)

The problem is that till now there was no UDP family, so logs expect an
AF_INET family to be passed for UDP there.

This patch manually remaps AF_CUST_UDP4 and AF_CUST_UDP6 to their "tcp"
equivalent that the log server parser expects. No backport is needed.