handle cleanup for signals we can catch

[radhermit]

For example, currently if a directory has to be created for a bind mount and then the process is sent a signal such as TERM or INT, we don't currently clean up the directory on exit.

[radhermit]

Cleanup handled for SIGINT/SIGTERM via pkgcore/snakeoil@f042a8c. Next up is resending the signals to the child namespace if the parent process exits due to receiving these signals.