Do not ignore singals

This fixes multiple issues:
  - hexcurse was running 100% CPU when the terminal window got closed,
    and the HUP signal was not handled, see
    https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=545711;
  - hexcurse was not exiting for SIGTERM;
  - it was not possible to put hexcurse into the background with SIGTSTP.