Actually handle mount options; readonly, xattr, atime, noexec, nosetuid.
Properly close (also do not take) z_mtime_vp when in readonly, so that
unmount will work.
Take out Linux unmount/remount calls for setting VFS options, darwin lets
you change them dynamically.

Still missing is taking mount arguments, ie, "mount -o ro" and
"import -o readonly". They are currently ignored.