hashbang.sh: Only use signed data for known_hosts
hashbang.sh: Only use signed data for known_hosts

This doesn't enforce that the keys are for hashbang.sh servers,
  but the previous implementation didn't do it properly either,
  and was vulnerable to appending unsigned data to the file.

Closes #71

KellerFuchs/hashbang.sh,hashbang/hashbang.sh,hashbang/hashbang.sh,KellerFuchs/hashbang.sh,hashbang/hashbang.sh,KellerFuchs/hashbang.sh,hashbang/hashbang.sh