hide mountpoints from nautilus

when building within a home directory, as /proc, /sys, etc are mounted and unmounted into the chroot at various points in the build, corresponding entries appear and disappear within the side panel of nautilus.

this is obviously undesirable.

use of -o x-gvfs-hide resolves this for the most part. i still see items occasionally pop up and having spent some time experimenting, i'm putting it down to buggy behaviour on the part of nautilus. (aside from those appearing when debootstrap is running - debootstrap also needs this fix).