Behave more like a regular POSIX utility

[kurahaupo]

Don't close stdin, even at end-of-file.
(Although this doesn't affect the code as it stands, it makes it brittle when incorporating into a larger program, where closing fd#0 could lead to it being reassigned to some other purpose.)

When '-' is used as the input or output name, use stdin or stdout instead (in line with POSIX guidelines).

[kolyshkin]

@kurahaupo can you please add a test case, checking that - works as input and output?

[kurahaupo]

@kolyshkin I can't find the existing tests of using stdin & stdout. If you could kindly point me at the relevant file(s) I will modify it(them) accordingly.