Enables bash strict mode

This casues the script to exit if any command returns false, stopping
the script on unexpected output.

Fixes some commands to stop them returning false if we don't care (ie
removing files that don't exist, creating directories that do exist and
unmounting things that are not mounted).
Makes the script executable
Fixes error when tar extracts to FAT

Makes tar not try to perserve ownership as FAT has no concept of
ownership.