Add option to chdir() before running the target program

[Wolf480pl]

It'd be useful, especially when using the -B option, to be able to change the working directory inside the sandbox before running the target program.

So that we could do something like sio2jail -b /my/empty/chroot:/:ro -b .:/tmp:ro --chdir /tmp -B ./abc instead of sio2jail -b /my/empty/chroot:/:ro -b .:/tmp:ro -B /tmp/abc

Or sio2jail -b /my/python/chroot:/:ro -b .:/tmp:ro --chdir /tmp -B python abc.py instead of PYTHONPATH=/tmp sio2jail -b /my/python/chroot:/:ro -b .:/tmp:ro -B python /tmp/abc.py