Only mmap() on regular files; mmap() on /dev/stdin will happily map as

Only mmap() on regular files; mmap() on /dev/stdin will happily map as
much as we ask for but only the first page will be usable. (We could get
the actual size with ioctl(FIONREAD) and mmap() that but it would need
to be done in the parent - I think just using read() is simpler.)