Lift the MOST_POSITIVE_FIXNUM/4 limitation on visited files (bug#8528).

Lift the MOST_POSITIVE_FIXNUM/4 limitation on visited files (bug#8528).

 src/fileio.c (Finsert_file_contents): Don't limit file size to 1/4
 of MOST_POSITIVE_FIXNUM.
 src/coding.c (coding_alloc_by_realloc): Error out if destination
 will grow beyond MOST_POSITIVE_FIXNUM.
 (decode_coding_emacs_mule): Abort if there isn't enough place in
 charbuf for the composition carryover bytes.  Reserve an extra
 space for up to 2 characters produced in a loop.
 (decode_coding_iso_2022): Abort if there isn't enough place in
 charbuf for the composition carryover bytes.