src/: Support for transport over IPv6 (and other protocol families).

We create a UDP socket for each distinct protocol family during
initialization, and then use the appropriate one for each server.  The
changes are somewhat invasive, but not really very complicated.

Signed-off-by: Mark Wooding <mdw@distorted.org.uk>
Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>