Simplify CTCP parsing by default

Rather than adhering to the CTCP 'specification', reply only to single
CTCP messages without text around them. All messages not fitting the
requirements are simply displayed in the buffer. The old behaviour can
still be enabled in the settings dialog.

Fixes #1130
Only escape \ with standards-compliant ctcp

Fixes #1142
Add support for SASL EXTERNAL

This is a method to identify to nickserv during connection registration,
like SASL PLAIN, but using the fingerprint of the certificate. Currently
the method used is chosen based on the mere presence of a certificate.
SASL PLAIN is not used as fallback (either this works or the certificate
is used as normal, with the race condition).
Name labels a bit nicer