insert-file-contents read size increase

insert-file-contents read size increase

This increases the max read size from 0.25 to 1 MiB, which is a
bit better for Emacs in my measurements.  The old value was taken
from coreutils, but was for a different purpose.
* src/fileio.c (INSERT_READ_SIZE_MAX): New constant, replacing
IO_BUFSIZE.  All uses changed.
(Finsert_file_contents): Do not use a read buffer that is larger
than INSERT_READ_SIZE_MAX.

(cherry picked from commit a8e46f11a89145390ee681e2dd14fbe6ab79b7f0)