Automatically enable "EchoList Only" areamgr mode when EchoList Keys are used

Logs a warning when doing so since this might be confusing if a sysop
specifically left this setting disabled ("Allow Users to Add Areas from Area
List" set to "Yes", the default, in echocfg) with EchoLists configured with
Required Keys.

This should automatically solve the "non-intuitive" configuration issue
reported via DOVE-net by poindexter FORTRAN (REALITY).