GDataInputStream: Clarify read_line() docs, mark as byte array

g_data_input_stream_read_line() and
g_data_input_stream_read_line_finish() don't do any encoding checks,
so we shouldn't call the returned value a "string" (which I'd like to
mean UTF-8).  Annotate them as byte arrays and add encoding warnings
to the docstrings.

https://bugzilla.gnome.org/show_bug.cgi?id=652758