Redmine #5973: Lower verbosity level of harmless message.

Signals are normal in cf-hub's lifetime, since the parent process may
kill children that are taking too long to complete (for example
because of saturated network). Detecting this is more appropriately
done in Mission Portal than an error message on the console.

Other than cf-hub, all agents are supposed to handle signals
gracefully, so it should not be displayed as an error.
Redmine #5973: Refactor TLS connection to time out rather than give up

On a slow network, we may get SSL_ERROR_WANT_READ/WRITE many times
from OpenSSL, due to non-blocking sockets. Previously we simply gave
the system one more chance to recover, but this is not sufficient
since the handshake can take more roundtrips than that.

Fix by basing the decision to give up on a timeout rather than number
of attempts.