* Added a 'DROPBOXES.FORCE_TO_MONO' field to the database.
* Incremented the database version to 268.
* 'RDDropbox::forceToMono()' and 'RDDropbox::setForceToMon()' methods
in 'lib/rddropbox.cpp' and 'lib/rddropbox.h'.
* Added a 'Force to Monaural' checkbox to the 'Edit Dropbox' dialog
in rdadmin(1).