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.

g_file_error_quark() is meant to be used with the GFileError enum
which does not correspond with errno, but must be converted with
g_file_error_from_errno().

At the same time, this removes g_strerror() use for
g_file_error_quark().