Terminate current GnuTLS connection for process PROC.
The connection should have been initiated using `gnutls-handshake'.
If CONT is not nil the TLS connection gets terminated and further
receives and sends will be disallowed. If the return value is zero you
may continue using the connection. If CONT is nil, GnuTLS actually
sends an alert containing a close request and waits for the peer to
reply with the same message. In order to reuse the connection you
should wait for an EOF from the peer.
This function may also return `gnutls-e-again', or `gnutls-e-interrupted'. (fn PROC CONT)