* cdl/synch.cdl: Added CYGIMP_MBOX_USE_MBOXT_PLAIN option. This is tested in various places but was not actually defined. It now is and defaults to 1 so that the plain version of mail boxes is selected. * include/mboxt.inl: * include/mboxt2.inl: Moved various CYG_ASSERTCLASS() calls to be within scheduler locked regions. Race conditions could have caused them to fail before. * tests/mbox1.cxx: * tests/kmbox1.cxx: Updated tests to work with mboxt implementation. This requires thread 1 to run at lower priority than thread 0.