res_monitor.c: Don't emit a warning about 'X' being unrecognized.

Code was added in 030f7d41 to warn if an unrecognized option was
passed to an application, but code in Monitor was taking advantage of
the fact that the application would silently accept an invalid option.

We now recognize the invalid option but we don't do anything if it's
set.

Fixes #639

(cherry picked from commit 6938b7f96244b3a2559650316c90eea16fdeed30)