Preliminary snap support

Snap packages nowadays get more directories configured for their purposes. Enable them to unblock users from using them on convergent tablets, ie install Docker, LXD or newer GIMP on their JingPads as requested often.

The new directories in question:

• /var/snap: Used for snap app data for deamons (IIRC)
• /var/cache/snapd: Cache directory
• /etc/udev/rules.d: Device access mediation
• /etc/modules-load.d: Enablement of loading missing kernel modules (which might be helpful depending on device port capabilities, ie list of kernel modules provided like on the JingPad)
• /root/snap: Snap app data for root

We have a precedent of enabling /etc/systemd/system for similar reasons.

Additionally /mnt gets reworked to stay a directory instead of a symlink to its Android counterpart directory. This way rbind-mounting into the Snap environment is possible, enabling installation and launching of apps the right way.