Add an extension of set_rcvar(), a new function set_rcvar_obsolete(),

Add an extension of set_rcvar(), a new function set_rcvar_obsolete(),
and $desc.

The set_rcvar_obsolete() is for displaying an obsolete variable
and the new one.  More specifically, a warning is displayed when
a variable is removed or changed in the source tree and the user
still defines the old one.

$router* and $ipv6_router* are replaced with $routed_* and
$route6d_* for consistency.  The old variables still work but
can be removed in the future.

MFC after:	3 days