CURLOPT: bump CURLPROXY_* enums to long, drop casts

This patch bumps the size of these macros from `int` to `long`, while
keeping their actual values the same. It may cause incompatibilities in
user code, requiring the bump of holder variables and/or adding casts:

- CURLPROXY_HTTP
- CURLPROXY_HTTP_1_0
- CURLPROXY_HTTPS
- CURLPROXY_HTTPS2
- CURLPROXY_SOCKS4
- CURLPROXY_SOCKS4A
- CURLPROXY_SOCKS5
- CURLPROXY_SOCKS5_HOSTNAME

Also:
- keep existing cast within the documentation to make sure it applies
  to older curl versions as well.

Closes #18054

lib/setopt.c

@@ -791,7 +791,7 @@ static CURLcode setopt_long(struct Curl_easy *data, CURLoption option,
      */
     if((arg < CURLPROXY_HTTP) || (arg > CURLPROXY_SOCKS5_HOSTNAME))
       return CURLE_BAD_FUNCTION_ARGUMENT;
-    data->set.proxytype = (unsigned char)(curl_proxytype)arg;
+    data->set.proxytype = (unsigned char)arg;
     break;
 
   case CURLOPT_PROXY_TRANSFER_MODE:

lib/url.c

@@ -2250,7 +2250,7 @@ static char *detect_proxy(struct Curl_easy *data,
  */
 static CURLcode parse_proxy(struct Curl_easy *data,
                             struct connectdata *conn, char *proxy,
-                            curl_proxytype proxytype)
+                            long proxytype)
 {
   char *portptr = NULL;
   int port = -1;
@@ -2568,7 +2568,7 @@ static CURLcode create_conn_helper_init_proxy(struct Curl_easy *data,
    * connection that may exist registered to the same proxy host.
    ***********************************************************************/
   if(proxy || socksproxy) {
-    curl_proxytype ptype = (curl_proxytype)conn->http_proxy.proxytype;
+    long ptype = conn->http_proxy.proxytype;
     if(proxy) {
       result = parse_proxy(data, conn, proxy, ptype);
       Curl_safefree(proxy); /* parse_proxy copies the proxy string */

lib/urldata.h

@@ -625,8 +625,7 @@ struct ip_quadruple {
 struct proxy_info {
   struct hostname host;
   int port;
-  unsigned char proxytype; /* curl_proxytype: what kind of proxy that is in
-                              use */
+  unsigned char proxytype; /* what kind of proxy that is in use */
   char *user;    /* proxy username string, allocated */
   char *passwd;  /* proxy password string, allocated */
 };
@@ -1435,7 +1434,7 @@ struct UserDefined {
   unsigned short proxyport; /* If non-zero, use this port number by
                                default. If the proxy string features a
                                ":[port]" that one will override this. */
-  unsigned char proxytype; /* what kind of proxy: curl_proxytype */
+  unsigned char proxytype; /* what kind of proxy */
   unsigned char socks5auth;/* kind of SOCKS5 authentication to use (bitmask) */
 #endif
   struct ssl_general_config general_ssl; /* general user defined SSL stuff */