Make stdout/stderr line buffered

This improves the behaviour with print()ing for debugging, when the
output is redirected to a file.

I was using `setbuf(…, 0)` initially, but it makes sense to buffer it
per line.  This uses `setvbuf` instead of `setlinebuf`, which might not
be available everywhere.

Closes https://github.com/awesomeWM/awesome/pull/267

awesome.c

@@ -331,6 +331,10 @@ main(int argc, char **argv)
         { NULL,      0, NULL, 0 }
     };
 
+    /* Make stdout/stderr line buffered. */
+    setvbuf(stdout, NULL, _IOLBF, 0);
+    setvbuf(stderr, NULL, _IOLBF, 0);
+
     /* clear the globalconf structure */
     p_clear(&globalconf, 1);
     globalconf.keygrabber = LUA_REFNIL;