tree-wide: (void)ify a few unlink() and rmdir()

Let's be helpful to static analyzers which care about whether we
knowingly ignore return values. We do in these cases, since they are
usually part of error paths.

src/sysusers/sysusers.c

@@ -244,7 +244,7 @@ static int make_backup(const char *target, const char *x) {
         return 0;
 
 fail:
-        unlink(temp);
+        (void) unlink(temp);
         return r;
 }