Making it easier to make filesystems jail-mountable

[James Gritton]

I've added a patch at https://reviews.freebsd.org/D14681 for anyone who 
wants to take a look.  While it doesn't (or at least shouldn't) change 
any current functionality, the point is to better automate the process 
of making a filesystem jail-aware.

Currently, this involves code changes in a few different places, mostly 
in expanding the list of allow.mount.* parameters.  With this change, 
that list still contains all such filesystems, but only if they're 
actually present in the kernel (including when KLDs are loaded).  The 
per-filesystem coding work is reduced to adding the VFCF_JAIL flag to 
the FS definition, and everything else is taken care of by the jail/vfs 
subsystems.

This originally came from a request to make NFS work with jails, which 
turns out to be a much bigger and unrelated thing, but still pointed me 
here.

- Jamie