FreeBSD 8.3+ support

This commit adds in FreeBSD 8.3+ support to the installer.

I've tested it on 8.3, 9.0 and 9.1, all RELEASE distributions, for both 2.0.0-p0 and 1.9.3-p194 MRI and both x86 and AMD64. It should work for other rubies since it's based off the OpenBSD support, which seems well tested, but I can't say I've tested any others. 9.0 AMD64 with 1.9.3-p194, my production environment, has the most love.

I've implemented dependency support (if your install has sudo) for package installation and I've tested local git clone and get.rvm.io install support (as of rys/rvm@513f8cdef705b8c4a5848bb2687400785a6b42a7).

It's a fairly trivial patch set, merges cleanly into current master and shouldn't affect any other platform support. I'll do my best to maintain it, at least for FreeBSD 9.0, from here on in.