server: Remove code to send recursive QUITs/SQUITs for non-QS servers.

Now that QS is required, this code can no longer be activated.
server: Simplify some code now ENCAP is mandatory.
capability: handle NULL passed to capability_get().
m_capab: fix a possible remote crash triggered by the CAPAB parsing code.
reference.conf: Correct description of general::client_exit.

It prefixes with "Quit: ", not "Client exit: ".
server: Move required CAPAB check after authentication and add snote and log.
server: Show the missing CAPABs when rejecting a server.
starttls: Don't send ERR_STARTTLS after successful STARTTLS.