Add throw_illegal_escape_char, add arg to throw_illegal_escape

Harmonize Illegal escape sequence \\%c in %Ss and
Illegal escape sequence %Ss error messages.
Also allow \? \

Note that the C standard requires such "invalid" escape sequences
to be diagnosed i.e., the compiler must print an error message.
[imcc] fix U type in sub fixup

Honor mk_const Slp, do not change S to U when called a 2nd time as S
with already stripped quotes.

Change -t8 private trace flag to -d2 proper imcc flag for mk_const tracking
Add more debugging verboseness.