We should no longer need to use our own versions of system header files

We should no longer need to use our own versions of system header files
when compiling on Linux, as we extracted from those copies the
definitions and declarations we need in order to dissect packets, and
put them into our own header files in the regular source directory.

We should also no longer need to define __STDC__ as 2 on Digital UNIX,
as we're no longer including the native OS's <netinet/ip.h> to get the
layout of an IP packet declared, we're using our own "ip.h" header file
for that.