netmap: pkt-gen: several updates from upstream

Various improvements to the netmap pkt-gen program:

• indentation fixes
• support for IPV6
• fixes to checksum computation
• support for NS_MOREFRAG
• rate limiting in ping mode

Reviewed by: bcr, 0mp
Approved by: gnn (mentor)
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D17698