Do not queue ignored commands

Do not queue ignored commands

Commands that will be ignored (such as 'xboard') are now recognized
with the easy commands, as it makes little sense to send them to the
engine thread just to be ignored. The 'accepted' and 'rejected' commands
are now also recognized and treated this way. In addition, commands are
now only echoed when they are queued, in debug mode: as the GUI thread
can only block on reading from the GUI we can be sure that all commands are
read as soon as they arrive, and echoing them just clutters the debug logs.