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 -'
`session_t::read_data` did not set context.pathname to `/dev/stdin`
for the special case `-f -`. I chose to adjust `source_context` too
as there is no sensible context if no file name is provided.
