digraphs: map private use characters to modern equivalents

RFC-1345 contains several characters in the private use block (for various
unofficial proposals at that time) which by now mostly have official Unicode
designations.  Using the character value instead of intended meaning is imho
stupid (as the digraphs don't make any kind of sense for most modern usage)
even though Vim and other adopters do so, probably unknowingly.

Try to convert these to suitable standard equivalents (going by character
names, Google, context, and some guesswork).