It's wrong to use std::system_category() for both GetLastError() and
errno on Windows.  Apparently, everybody uses std::generic_category()
for errno values, which appears to be a safe choice.

Some discussion on this confusing topic can be found here:

 https://stackoverflow.com/questions/28746372/system-error-categories-and-standard-system-error-codes

Reduce header dependencies.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>

Remove the "whence" parameter that is not actually necessary, and only
complicates the InputStream implementations.

Replaces GLib's GError.

Implement the "scan_stream" method that can read tags from any
input_stream object.  This requires a FLAC__IOCallbacks implementation
based on the input_stream API.