mmap in php_passthru_fd in file.c ?

If a system has mmap() a readfile() will mmap the entire file to memory and then dump that while without mmap it will read it one block at a time. That's a siginificant memory difference and one that may not be expected. Obviously the mmap will be faster, but if as in bug #10701, someone is adding headers or doing something else to really large files, things are going to break. Since readfile() already has an opotional argument I think the right approach is a separate function that turns the mmap off and reads the file block by block. fpassthru() doesn't have an optional arg, so we could toggle it there for that function. Comments? -Rasmus