Seekable file_descriptor_source /sink?

[gmsb]

The supplied file_descriptor_source and file_descriptor_sink are not seekable. Is this an oversight?

For file_descriptor_source, all that appears to be needed is changing from:

    struct category : public source_tag, closable_tag { };

to:

    struct category
      : input_seekable,
        device_tag,
        closable_tag
    { };

and adding:

    using file_descriptor::seek;

Similarly for file_descriptor_sink.

[Marshall Clow]

assigning to actual user "bemandawes" instead of unknown user "beman_dawes"

[Jonathan Turkanis]

(In [42268]) fixed #823 and added a test case