Simplify has_beginning_of_path()
main.c: Fix comment and formatting.
Rework unmount_directories and shared_directories.

Make everything dynamic so we can have command line options for
them in the future.
Add --keep-shm option to share /dev/shm/ with the host.

This will be useful in the future when we want to have access
to pulseaudio from within the jail.
Add --shared and --unmount options.