Add rc.conf variables to control host key length

[Garrett Wollman]

<<On Sun, 24 Jun 2012 14:52:17 -0400, "J. Hellenthal" <jhellenthal at dataix.net> said:

> 2048 is well more than efficient. Speaking soley for RSA in that matter.

I asked R. about that a few months back, and he expressed the view
that 2,048 bits is the *minimum* RSA key size anyone should consider
using at this point.  I'm willing to take his word for it.

-GAWollman