Introduce C-h command dispatch function

Introduce C-h command dispatch function

Replace role of show-next-page with C-h-dispatch which immediately reads
a key and calls a command from C-h-map, which may be one of several
paging commands, a command to undo the last keypress, or a command to
directly access describe-prefix-bindings.

This commit does not include documenting these changes in the readme.

Note that several options become deprecated here as they no longer have
the same (if any effect).