hide mountpoints from nautilus

during execution, 'proc' and 'sys' entries appear in the nautilus side panel corresponding to mounts of /proc and /sys respectively.

this is obviously undesirable.

use of the option x-gvfs-hide should fix this. it is apparently not needed when doing a bind mount, so i have not applied it in those cases.

i noticed this creating live images with live-build, for which the same fix has been proposed, but requires the change to be applied in debootstrap also to be complete.

as mentioned in the MR for live-build the solution seems to be somewhat flaky with such entries still appearing occasionally during a run of live-build, but at far fewer points. after some experimentation granularly executing substages and manually trying mount directly i could not narrow down any problem other than apparent flakiness in nautilus obeying the x-gvfs-hide property. (both with instances of x-gvfs-hide use and one using --bind).

please note that i have not tested this.