Add mechanism preventing loading sbo-shell multiple times, add _restart

[jarmuszz]

When sbo-shell starts set some variable, when it ends unset it. At the end of the file check whether the said variable is set or not.

The _restart function will be a lot more tricky - because sbo-shell is suposed to be sourced, the $0 is equal to shell's $0, and ergo, sbo-shell doesn't know it's path.