Enable full buffering on stdout/stderr.

Now that we explicitly flush on each line, we can enable this to reduce
the number of write() calls.

ui/main.c

@@ -342,6 +342,9 @@ int main(int argc, char **argv)
 	struct cmdline_args args = {0};
 	int ret = 0;
 
+	setvbuf(stderr, NULL, _IOFBF, 0);
+	setvbuf(stdout, NULL, _IOFBF, 0);
+
 	opdb_reset();
 	ctrlc_init();