Remove $FreeBSD$: one-line .c comment pattern

Remove /^/[*/]\s*\$FreeBSD\$.*\n/

nemtap: lb app: Validate ihl field when hashing packet

MFC after: 1 week

netmap: lb: switch to libnetmap

Use the newer libnetmap (included in base) rather than the older
nm_open()/nm_close() defined in netmap_user.h

MFC after: 3 days

netmap: tools: fix several compiler warnings

MFC after: 1 week

netmap: add load balancer program

Add the lb program, which is able to load-balance input traffic
received from a netmap port over M groups, with N netmap pipes in
each group. Each received packet is forwarded to one of the pipes
chosen from each group (using an L3/L4 connection-consistent hash function).
This also adds a man page for lb and some cross-references in related
man pages.

Reviewed by: bcr, 0mp
Approved by: gnn (mentor)
Differential Revision: https://reviews.freebsd.org/D17735