libifconfig: Add function to get bridge status

The new function operates similarly to ifconfig_lagg_get_lagg_status and
likewise is accompanied by a function to free the bridge status data structure.

I have included in this patch the relocation of some strings describing STP
parameters and the PV2ID macro from ifconfig into net/if_bridgevar.h as they
are useful for consumers of libifconfig.

Reviewed by: kp, melifaro, mmacy
Approved by: mmacy (mentor)
MFC after: 1 week
Relnotes: yes
Differential Revision: https://reviews.freebsd.org/D25460