libifconfig: Fix bridge status member list

When this functionality was moved to libifconfig in 3dfbda3401abea84da9,
the end of list calculation was modified for unknown reasons, practically
limiting the number of bridge member returned to (about) 102.

This patch changes the calculation back to what it was originally and
adds a unit test to verify it works as expected.

Reported by: Patrick M. Hausen (via ML)
Reviewed by: kp
Approved by: kp
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D43135

Remove $FreeBSD$: two-line .h pattern

Remove /^\s*\*\n \*\s+\$FreeBSD\$$\n/

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