libfetch: Avoid SIGPIPE on network connections.

To avoid unexpected process termination from SIGPIPE when writing to a
closed network connection, enable SO_NOSIGPIPE on all network connections.

The POSIX standard MSG_NOSIGNAL is not used since it requires modifying all
send calls to add this flag. This is particularly nasty for SSL connections.

Reviewed by:	des
Tested by:	bapt
MFC after:	5 days

lib/libfetch/common.c

@@ -209,11 +209,13 @@ conn_t *
 fetch_reopen(int sd)
 {
 	conn_t *conn;
+	int opt = 1;
 
 	/* allocate and fill connection structure */
 	if ((conn = calloc(1, sizeof(*conn))) == NULL)
 		return (NULL);
 	fcntl(sd, F_SETFD, FD_CLOEXEC);
+	setsockopt(sd, SOL_SOCKET, SO_NOSIGPIPE, &opt, sizeof opt);
 	conn->sd = sd;
 	++conn->ref;
 	return (conn);