Improved M4 quotation

Improved M4 quotation

This fixes M4 quotation of certain strings beginning with `yy` (in
section 3 of the input file only) and character literals.  The new
quotation method is also less brittle and faster.

Tests that relied on the old behavior were fixed.

Also, `yyconst` is no longer defined; use `const` (which it
unconditionally was defined to) instead.