Use C version of LWORD in INTERPRET

* csrc/pf_words.c (ffLWord): New.
(Word): Factored out from ffWord.
(ffWord): Call Word.

* csrc/pfcompil.c (ffInterpret): Use ffLWord instead of ffWord to
preserve case for NUMBER?.  Can't use ffWord because that would
convert a character written as "'a'" to "'A'".

* csrc/pfcompil.h (ffLWord): Define prototype.

* fth/numberio.fth: Fix typso.