Rewrote error & log functions to be more general, use longjmp to get out of

trouble and into predictable cleanup functions (one for each of serve,
client & control contexts).  We use 'fatal' to mean 'kill the thread' and
'error' to mean 'don't kill the thread', assuming some recovery action,
except I don't use error anywhere yet.

src/serve.h

@@ -22,6 +22,8 @@ enum mirror_finish_action {
 
 struct mirror_status {
 	pthread_t            thread;
+	/* set to 1, then join thread to make mirror terminate early */
+	int                  signal_abandon;
 	int                  client;
 	char                 *filename;
 	off64_t              max_bytes_per_second;