Allow opening device files and FIFOs with file:open/2

To the best of our knowledge this was introduced since file operations
on device files/FIFO:s could hang the emulator forever back when the
emulator was single-threaded and lacked IO threads; a read operation
could block all progress preventing the write operation it waited for
from occurring.

Granted, this could still happen through starving all dirty IO
schedulers, but the same issue can arise with NFS files which we've
always allowed.

Removing this restriction also lets us remove a stat(2) call that was
added to specifically allow `/dev/null`.

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