Special handling of signals for cf-key.

Handle SIGINT and SIGTERM to set gobal flag that makes the
ShowHost callback return false, thus causing ScanLastSeenQuality break
out of the while loop and return cleanly.

Fixes redmine #5957
Merge pull request #1742 from vohi/redmine/5957

Special handling of signals for cf-key.
Satisfy linker when building unit test.

This test is not testing anything right now, probably needs to
link in more code from libpromises once it does, but for now keep
it simple.
Partial revert of 0a97e68c48 (Special handling of signals for cf-key)

In that commit, the signal handler for SIGPIPE was removed. SIGPIPE
can easily happen when you pipe your input into for example grep, so
we need to restore this. We ignore the signals so any output after the
signal will just go to the void.
examples/container_key_iteration.cf: add a container key iteration example
Be silent by default if peer sends few bytes. Redmine #5919.

So that cf-serverd does not complain when portscanned,
e.g. selectservers() policy function.
Merge pull request #1743 from jimis/master

Be silent by default if peer sends few bytes. Redmine #5919.
Lower the time spent scanning lastseen DB.

The total DB access time will thus not depend on the time spent running the callback (it seems to be huge in the case of reverse DNS resolution).

This commit makes it easier to later convert the scan into an RDONLY operation.
Merge pull request #1744 from jeffali/fix/scankeys

Lower the time spent scanning lastseen DB.
cf-key -s has its special sig handler but the other operations need one too
Merge pull request #1745 from jeffali/cf_key/generic_sig

cf-key -s has its special sig handler but the other operations need one too
Update changelog