CURLOPT_DEFAULT_PROTOCOL: added

- Add new option CURLOPT_DEFAULT_PROTOCOL to allow specifying a default
protocol for schemeless URLs.

- Add new tool option --proto-default to expose
CURLOPT_DEFAULT_PROTOCOL.

In the case of schemeless URLs libcurl will behave in this way:

When the option is used libcurl will use the supplied default.

When the option is not used, libcurl will follow its usual plan of
guessing from the hostname and falling back to 'http'.

src/tool_paramhlp.c

@@ -338,6 +338,27 @@ long proto2num(struct OperationConfig *config, long *val, const char *str)
   return 0;
 }
 
+/**
+ * Check if the given string is a protocol supported by libcurl
+ *
+ * @param str  the protocol name
+ * @return PARAM_OK  protocol supported
+ * @return PARAM_LIBCURL_UNSUPPORTED_PROTOCOL  protocol not supported
+ * @return PARAM_REQUIRES_PARAMETER   missing parameter
+ */
+int check_protocol(const char *str)
+{
+  const char * const *pp;
+  const curl_version_info_data *curlinfo = curl_version_info(CURLVERSION_NOW);
+  if(!str)
+    return PARAM_REQUIRES_PARAMETER;
+  for(pp = curlinfo->protocols; *pp; pp++) {
+    if(curlx_raw_equal(*pp, str))
+      return PARAM_OK;
+  }
+  return PARAM_LIBCURL_UNSUPPORTED_PROTOCOL;
+}
+
 /**
  * Parses the given string looking for an offset (which may be a
  * larger-than-integer value). The offset CANNOT be negative!