Wire protocol encoding for 'inet' type is defined unreliably

Hi,

I noticed today that the network encoding of the 'inet' type uses values
derived from the system header file <sys/socket.h> to tag whether the
address is IPv4 or IPv6.

In particular, <src/include/utils/inet.h> defines PGSQL_AF_INET to be
AF_INET, which is 2 on my system.  Wouldn't it be better to use a more
constant constant for this purpose?

--
Duncan Smith