[imcc] document .lex "" limitation. add -d2 for MKCONST

Handle GH #1095 .lex corner cases with double-quotes.
Do not use it until refactored.

Add a new -d2 flag to print mk_const strings, useful to find
about quoting and escaping, or not. Was used for LEXER some years ago.
[test] todo .lex "" testcase GH #1095