Recognize DCC SSEND when receiving files in erc-dcc

Recognize DCC SSEND when receiving files in erc-dcc

* lips/erc/erc-dcc.el (erc-dcc-open-network-stream): Use TLS
for new connections when :secure flag is set.
(erc-dcc-do-GET-command): Set secure flag when user explicitly passes
an "-s" option.
(erc-dcc-do-LIST-command): Show an "s" to indicate a secure connection
when applicable.
(erc-dcc-query-handler-alist): Add extra items for "SSEND", etc.
(erc-dcc-handle-ctcp-send): Set secure flag when a leading "S" appears
in the command type.