netmap: pkt-gen: sync with upstream

Keep in sync with the recent upstream changes:

Fix compilation on 32-bit architectures
Update IP length, UDP length/checksum when size changes
Man page fixes

Submitted by: jlduran@gmail.com
MFC after: 7 days
Differential Revision: https://reviews.freebsd.org/D39760

(cherry picked from commit 8c3b8c838674239538682b914bc1d7059255f116)