Removing rc-mount.sh script and all the references to it. Replacing the do_unmount shell function with the do_unmount.c program.

The new program implements the same functionalities with the added feature of parallel unmounting when possible:

• Unmount every mount point in a different thread.
• Wait for every mount point in a subdirectory to be unmounted before proceeding with the current one.
• Never unmount two shared mount points at the same time.
• Check if shared mount point is still present before unmounting.
• Retrieve list of shared mount points from /proc/1/mountinfo.

Fixes: #662