MEDIUM: ssl: Add splicing with SSL.

MEDIUM: ssl: Add splicing with SSL.

Implement the splicing methods to the SSL xprt (which will just call the
raw_sock methods if kTLS is enabled on the socket), and properly report
that a connection supports splicing if kTLS is configured on that
connection.
For OpenSSL, if the upper layer indicated that it wanted to start using
splicing by adding the CO_FL_WANT_SPLICING flag, make sure we don't read
any more data from the socket, and just drain what may be in the
internal OpenSSL buffers, before allowing splicing