v8.1

Rename some derived files

Names like tunebook-tabs.pdf made sense once upon a time when
this file was the only one containing tabs, but are now looking
silly in the face of lots of different sorts of tabs. Likewise
tunebook-dulcimer.pdf now only contains Dulcimer chords while
tunebook-dulcimer-tabs-dad.pdf contains chords and tabs.

This edit renames several output files to be more specific (and
to include tunings for Whistle and Dulcimer), renames some internal
support files to match, and adds redirects from the old output file
names to the new ones.