Improve FreeBSD support

Quagga FreeBSD port used to include a set of patches to make it work. Quagga-RE includes commits, which either merge or supersede those patches. This work has been tested and merged into master. There are also other changes that fixes FreeBSD-specific problems, and some more changes is expected in the future. (Other platforms may also benefit from those changes.)

mapping of commits

bug ID	summary	RE-testing-0.99 commit	master commit
408	configure: fix FreeBSD header detection	5112233b7929299b5200ef820d1567f5d7db50e6	2b43bf231c3004efaef96f2b85d0e0f4cf6a1ea1
	configure: fix OpenPAM detection	4a1342a56c6fdb90d9af3109afa9442ff0b850d6	6159928d28285938356a3d48d2477fbdb7890e61
	lib: simplify interface of setsockopt_multicast_ipv4().	bf510e9bc06b55ae9d9c1d6ed67152b94e80977e	69bf3a39931ce61a92fb04d8888b4655fd5620a2
	lib: fix omission in the previous commit to lib/sockopt.c	87d29037b3f390cfb9d4e9e2aef4f0b49f98d557	e0afa6f44a77a89a543a03284c9f63af7b057c08
	lib: futher simplification of setsockopt_ipv4_multicast()	96a0a39261b5a848228f096a1a1836fa12ed3e69	ee7e75d38bdb139b10f3a1d35209758409db7fb5
420	lib: use "protocol-independed API" from RFC3678, if that is available	d6af701250915fdc7449500d9191f4eb2e6cfa4d	10d04cdb4198b69073ea0cdd4c26a64d6697b33e
	configure: test for glibc backtrace even without glibc.	aa4edc594aaef69bd5ef5b25c63367710a08ff66	eda3529e9cf163ccbcf47c889b16fd707d2aed76
	vtysh: [build] silence preprocessor errors in some cases	e584fa06a1fbee4b9e3fe1aa6099566608d54964	566b36ebf903fd873c974ba34c19097d8fb48ce4