Added alternative alphabet

The menu option "cfg regn"/"digt fmt" is now "cfg regn"/"text fmt".
Under "text fmt" there is now an option to select an alternative
alphabet with a preference for capital letter forms.
Improved piezo control

The code to control volume of lower frequencies now exploits the piezo
element's resonant frequency to set volume.  The effect is to maximize
sound pressure at the highest volume level.

More careful control of Timer/Counter 1 when generating frequencies
has eliminated the faint clicking noise at low volume.