sbin/ifconfig: Use libifconfig to get lagg status

[freqlabs]

Summary

Also trimmed an unused block of code that never prints out LAGG_PROTOS.

Test Plan

# ifconfig > /tmp/a
# ifconfig -v > /tmp/a_v
# git stash pop && make -C sbin/ifconfig && make -C sbin/ifconfig install
<snip>
# ifconfig > /tmp/b
# ifconfig -v > /tmp/b_v
# cmp /tmp/a /tmp/b && echo ok
ok
# cmp /tmp/a_v /tmp/b_v && echo ok
ok