Don't use mmap() for non-regular files, since st_size is only meaningful

Don't use mmap() for non-regular files, since st_size is only meaningful
for regular files.  This fixes recent breakage of cp'ing from /dev/zero.
/dev/zero doesn't support mmap(), but the device driver mmap routines are
not called for mapping 0 bytes, so the error was not detected.  mmap()
can't even be used for cp'ing special files that support mmap(), since
there is general way to determine the file size.