Use unsigned values in tok2str and bittok2str routines.

Use unsigned values in tok2str and bittok2str routines.

This prevents the compiler issue mentioned in GitHub issue #451, and
also cleans up some other signed vs. unsigned stuff.

While we're at it, clean up bittok2str_internal() (just pass it the
separator string, not a Boolean value that's tested to choose the
separator string), and print unknown arguments to the bittok2str
routines in hex, not decimal.