cp: port better to old limited hosts

cp: port better to old limited hosts

Port better ancient platforms where OFF_T_MAX is only 2**31 - 1,
but some devices have more than that many bytes.
* src/copy-file-data.c (copy_file_data): Byte count is now
count_t, not off_t.  All callers changed.  Since we need to check
for overflow anyway, also check for too-small calls to fadvise.