Handle very large -f files by rejecting them.

Handle very large -f files by rejecting them.

_read(), on Windows, has a 32-bit size argument and a 32-bit return
value, so reject -f files that have more than 2^31-1 characters.

Add some #defines so that, on Windows, we use _fstati64 to get the size
of that file, to handle large files.

Don't assume that our definition for ssize_t is the same size as size_t;
by the time we want to print the return value of the read, we know it'll
fit into an int, so just cast it to int and print it with %d.