Set a user-agent for curl isnewer

Fix sync warning about checking update failed due to rejected
connection without a user-agent on some mirrors.

The user-agent for curl isnewer is conform https://trac.macports.org/ticket/40319.

Closes: https://trac.macports.org/ticket/70973

src/pextlib1.0/curl.c

@@ -836,6 +836,7 @@ CurlIsNewerCmd(Tcl_Interp* interp, int objc, Tcl_Obj* const objv[])
 		CURLcode theCurlCode;
 		long theModDate;
 		long userModDate;
+		const char* userAgent = PACKAGE_NAME "/" PACKAGE_VERSION " libcurl/" LIBCURL_VERSION;
 
 		optioncrsr = 2;
 		lastoption = objc - 3;
@@ -906,6 +907,13 @@ CurlIsNewerCmd(Tcl_Interp* interp, int objc, Tcl_Obj* const objv[])
 			break;
 		}
 
+		/* -A option */
+		theCurlCode = curl_easy_setopt(theHandle, CURLOPT_USERAGENT, userAgent);
+		if (theCurlCode != CURLE_OK) {
+			theResult = SetResultFromCurlErrorCode(interp, theCurlCode);
+			break;
+		}
+
 		/* --max-redirs option, same default as curl command line */
 		theCurlCode = curl_easy_setopt(theHandle, CURLOPT_MAXREDIRS, 50);
 		if (theCurlCode != CURLE_OK) {