gdatainputstream: Deprecate read_until() in favour of read_upto()

g_data_input_stream_read_upto() was introduced in 2.26; now it’s GLib
2.56, we can probably deprecate the old versions (since the handling of
consuming the stop character differs between the sync and async versions
of it).

Signed-off-by: Philip Withnall <withnall@endlessm.com>

https://bugzilla.gnome.org/show_bug.cgi?id=584284