protocol handler: added flags field

The protocol handler struct got a 'flags' field for special information
and characteristics of the given protocol.

This now enables us to move away central protocol information such as
CLOSEACTION and DUALCHANNEL from single defines in a central place, out
to each protocol's definition. It also made us stop abusing the protocol
field for other info than the protocol, and we could start cleaning up
other protocol-specific things by adding flags bits to set in the
handler struct.

The "protocol" field connectdata struct was removed as well and the code
now refers directly to the conn->handler->protocol field instead. To
make things work properly, the code now always store a conn->given
pointer that points out the original handler struct so that the code can
learn details from the original protocol even if conn->handler is
modified along the way - for example when switching to go over a HTTP
proxy.

lib/tftp.c

@@ -196,7 +196,8 @@ const struct Curl_handler Curl_handler_tftp = {
   ZERO_NULL,                            /* perform_getsock */
   tftp_disconnect,                      /* disconnect */
   PORT_TFTP,                            /* defport */
-  PROT_TFTP                             /* protocol */
+  PROT_TFTP,                            /* protocol */
+  PROTOPT_NONE                          /* flags */
 };
 
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