pcoin_keys fixed

Thanks to ~jmjl, I was able to find that pcoin_keys didn't work. To fix it, tcoin uses sneaky magic but with stdin isntead of an argument, and a new library imported called popen2, that lets me send both stdin and receive stdout, to successfully get "ls <path to the pcoin_keys directory>" to work (it execl's another instance of the tcoin comand so that it runs under setuid instead of as the original user who ran it.