alpm: Fix SIGINT handling re: aborting download

Upon receiving SIGINT a flag is set to abort the (curl) download.
However, since it was never reset/initialized, if a front-end doesn't
actually exit on SIGINT, and later tries any operation that needs to
perform a new download, said download would always get aborted right
away due to the flag not having been reset.

(cherry picked from commit ffde85aadf)

lib/libalpm/dload.c

@@ -431,6 +431,7 @@ static int curl_download_internal(struct dload_payload *payload,
 	/* Ignore any SIGPIPE signals. With libcurl, these shouldn't be happening,
 	 * but better safe than sorry. Store the old signal handler first. */
 	mask_signal(SIGPIPE, SIG_IGN, &orig_sig_pipe);
+	dload_interrupted = 0;
 	mask_signal(SIGINT, &inthandler, &orig_sig_int);
 
 	/* perform transfer */