Fix GCC warnings with _FORTIFY_SOURCE=2

- Decleare some return values with (void)fn(), for cases where
  we don't care (dropping table headers), or best effor
- Check return value from fgets() and chdir() in some cases that
  are valid, i.e., continuing execution is pointless

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

src/cgroup.c

@@ -54,7 +54,7 @@ void cgroup_init(void)
 	}
 
 	/* Skip first line, header */
-	fgets(buf, sizeof(buf), fp);
+	(void)fgets(buf, sizeof(buf), fp);
 	while (fgets(buf, sizeof(buf), fp)) {
 		char *cgroup;
 		char rc[80];