Fix int/EMACS_INT usage in fns.c.

Fix int/EMACS_INT usage in fns.c.

 fns.c (Fcompare_strings, Fstring_lessp, concat)
 (string_make_unibyte, Fstring_as_unibyte, Fsubstring)
 (Fsubstring_no_properties, substring_both, Ffillarray)
 (Fclear_string, mapcar1, Fmapconcat, Fmapcar, Fmapc)
 (Fbase64_encode_region, Fbase64_encode_string, base64_encode_1)
 (Fbase64_decode_region, Fbase64_decode_string, base64_decode_1)
 (Fmd5): Use EMACS_INT for buffer and string positions and length
 variables and arguments.
 <struct textprop_rec>: Use EMACS_INT for positions.
 lisp.h (substring_both): Adjust prototype.