MFC r340279

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

MFC r339659

netmap: add man page for the bridge program

Added bridge(8).
Also, minor fixes to the netmap "bridge" application:
- indentation fixes and code cleanup
- better usage description
- better processing of netmap flags

Reviewed by: 0mp
Approved by: gnn (mentor)
Differential Revision: https://reviews.freebsd.org/D17664