Make admin port only reachable from localhost by default (127.0.0.1:4985)

This prevents the potentially-catastrophic error where somebody doesn't
realize they have to block port 4985, and exposes the admin API to the
public.
Of course you can override this with e.g. -adminInterface=:4985
Merge commit '506254baaec1c8ff232e51d66895216acafefc50' into stable