added DSP "nag" test mode. instead of waiting for an IRQ, it
periodically commands the DSP to play another block even if it's already
playing one (Triton Crystal Dream style).
update nag mode so that it can be disabled, or enabled for non-hispeed
transfers, or for normal and highspeed (which is not recommended).
updated code to indicate proper "supported" status for cases where no
IRQ is assigned, but DSP nagging and ack polling are available.
sndsb library: goldplay timer tick functions now also set timer tick
flag. added code to assign a const char string message to give the user
interface a textual string explaining what's wrong with the settings.
Modified help dialogs and added a 3rd explaining what the new settings
mean.