Fix for rename of MAG_ENABLE to COMPASS_ENABLE for ArduPilot master

The MAG_ENABLE parameter is renamed in master to COMPASS_ENABLE. Some users are confused by the greyed out checkbox on the Compass setup page "Enable compasses".
Can we get that updated to cope with both names?

PS: I initially submitted this issue thinking MAG_ENABLE was gone, then realised it was just renamed

Start at the Compass setup page and search the codebase for MAG_ENABLE and COMPASS_ENABLE. Update the "Enable compasses" checkbox handling so it works with both parameter names, then verify that the checkbox is no longer incorrectly greyed out for ArduPilot master.

Written by the indexing model from the issue text.