Protocols sending the OS's AF_INET6 value are OS-specific or broken.

Protocols sending the OS's AF_INET6 value are OS-specific or broken.

Check for all the different AF_INET6 values that are on various OSes.
If Totem is, and will forever be, used *ONLY* on one particular OS, feel
free to remove the uses of other _AF_INET6 values (but do *not* change
back to using the OS's AF_INET6; this should dissect the protocol
correctly on *all* OSes).

Add a common AF_INET definition to epan/aftypes.h while we're at it, and
use that; as most OSes picked up 4.2BSD's AF_INET value, most if not all
of them use 2, but IPv6 came out after 4.2BSD, and various OSes all
picked their own values for AF_INET6.

Change-Id: Iae15dfdd15203ed3ecd078a6499821dc09139a98
Reviewed-on: https://code.wireshark.org/review/2458
Reviewed-by: Guy Harris <guy@alum.mit.edu>