tls: more secure default for GNU TLS

This does not cover all cases since only applications using
`open-gnutls-stream` will be "secure". Those using `gnutls-negotiate`
will still have to pass the appropriate options.

See:
 http://debbugs.gnu.org/cgi/bugreport.cgi?bug=13374
erc: use znc to connect to ZNC

We try to get password from auth source instead of putting it in the
configuration file.
znc: provide network name in password

We use the same ZNC server for many networks. In this case, the
password should be `login/network:password`. This is not something
that ZNC.el is handling. Therefore, we put the network name into the
login and we redefine `znc-erc-ssl-connector` to retrieve this network
name and build the correct password.

A cool side-effect is that the password is no longer displayed in the
messages.