ip.c already works on OpenBSD, it's just missing some headers:

Make use of strerror(errno) and format all errors equally:
function ['parameters']: error message

Replace warn() and warnx() with fprintf() and add <stdio.h> where
necessary.

Replace #ifdef with #if defined() and #elif with #elif defined() as it
should only test if it is defined or not.

This brings us a lot more tidiness.

For multiple reasons the program is now split:
- Make future porting to OpenBSD easier
- Assign header includes to individiual functions
- Make future program extensions easier
- Recompile only changed parts