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.