Add g_data_input_stream_read_upto{,async,finish}
These functions are meant to replace the read_until() flavour, with the
following improvements:
- consistency between the synchronous and asynchronous versions as to
if the separator character is read (it never is).
- support for using a nul byte as a separator character by way of
addition of a length parameter which allows stop_chars to be treated
as a byte array rather than a nul-terminated string.
The read_until() functions are not yet formally deprecated, but a note
has been added to the documentation warning not to use them as they will
be in the future.
This is bug #584284.