change handling of standard input

For `-f /dev/stdin`, the `pathname` of the parsing context will be empty
as for any other streamed input.

`instance_t::include_directive` did not work as expected for `-f /dev/stdin`
and relative file names. One would expect them to be relative to the current
directory rather than `/dev`.

This will lead to `While parsing file ""` messages. This could be
adjusted to read `While parsing standard input`, but maybe it's not
worth the special cases.

This commit also fixes bug 788: behaviour of source_context for '-f -'

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