Misc fixes

Use read's built-in readline capabilities, present the default to the user for editing
I think Bash-isms are the way to go since this script (and all scripts shipped with LXC) rely on Bash.

removed the destroy() function since "destroying" a container is trivial and the function didn't work anyway.
+ moved /tmp onto tmpfs

One more thing: is it necesary for $? checks to return to the calling function or can we stop the script entirely? This would make the code *A lot* shorter.