protocol source, all about protocols and uri schemes

Add protocol.h and protocol.c containing all about libcurl's
known URI schemes and their protocol handlers (so they exist).

Moves the scheme definitions from the various sources files into
protocol.c. Schemes are known and used, even of the protocol
handler is not build or just not implemented at all.

Closes #20906

lib/pop3.c

@@ -1691,7 +1691,7 @@ static CURLcode pop3_setup_connection(struct Curl_easy *data,
 /*
  * POP3 protocol.
  */
-static const struct Curl_protocol Curl_protocol_pop3 = {
+const struct Curl_protocol Curl_protocol_pop3 = {
   pop3_setup_connection,            /* setup_connection */
   pop3_do,                          /* do_it */
   pop3_done,                        /* done */
@@ -1712,37 +1712,3 @@ static const struct Curl_protocol Curl_protocol_pop3 = {
 };
 
 #endif /* CURL_DISABLE_POP3 */
-
-/*
- * POP3 protocol handler.
- */
-const struct Curl_scheme Curl_scheme_pop3 = {
-  "pop3",                           /* scheme */
-#ifdef CURL_DISABLE_POP3
-  ZERO_NULL,
-#else
-  &Curl_protocol_pop3,
-#endif
-  CURLPROTO_POP3,                   /* protocol */
-  CURLPROTO_POP3,                   /* family */
-  PROTOPT_CLOSEACTION | PROTOPT_NOURLQUERY | /* flags */
-  PROTOPT_URLOPTIONS | PROTOPT_SSL_REUSE | PROTOPT_CONN_REUSE,
-  PORT_POP3,                        /* defport */
-};
-
-/*
- * POP3S protocol handler.
- */
-const struct Curl_scheme Curl_scheme_pop3s = {
-  "pop3s",                          /* scheme */
-#if defined(CURL_DISABLE_POP3) || !defined(USE_SSL)
-  ZERO_NULL,
-#else
-  &Curl_protocol_pop3,
-#endif
-  CURLPROTO_POP3S,                  /* protocol */
-  CURLPROTO_POP3,                   /* family */
-  PROTOPT_CLOSEACTION | PROTOPT_SSL | /* flags */
-  PROTOPT_NOURLQUERY | PROTOPT_URLOPTIONS | PROTOPT_CONN_REUSE,
-  PORT_POP3S,                       /* defport */
-};