* more compiler warning fixes for the lang...utils subdirectories

  (based on amd64 recompile)
* move externs in C files to ptexlib.h/&lua/luatex-api.h for now
  (for most, this is not the final place yet)
* move typedef const_lstring to stringpool.h
  (used in pdfobj.c)
* typecast patch for kpathsea/lib.h and cpascal.h 
  (probably temporary, PEB is likely to change xmalloc's prototype)