Improved --help information relating to clean up hooks.
Removed shell globbing code from find-regex-escape() as it is unnecessary as quoting the variable prevents shell globbing.
Run hooks is sub-shell. This isolates hooks from each other and unrarall because

* A hook cannot change the current working directory of unrarall
* A hook cannot modify the global variables of unrarall.

This should make unrarall more robust to badly coded hooks.

This is completly untested but it "should" work...