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.
Fix manpage

The manpage was issuing warnings, see for yourself with:
  man --warnings -l man/hexcurse.1 >/dev/null

The removal of the .SP lines doesn't change the formatting, it seems.
Merge branch 'master' of https://github.com/nilcons/hexcurse into nilcons-master

PR submitted by @nilcons resolves https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=545711