Select lacp egress ports based on NUMA domain

This change creates an array of port maps indexed by numa domain
for lacp port selection. If we have lacp interfaces in more than
one domain, then we select the egress port by indexing into the
numa port maps and picking a port on the appropriate numa domain.

This is behavior is controlled by the new ifconfig use_numa flag
and net.link.lagg.use_numa sysctl/tunable (both modeled after the
existing use_flowid), which default to enabled.

Reviewed by: bz, hselasky, markj (and scottl, earlier version)
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D20060