ADDED: PL_get_stream(), replacing PL_get_stream_handle() and adding a
flags argument.
Document that Sclose deals safely with an erased stream.
MODIFIED: do not complain if close/1 is called on a stream pair of
which one or both streams are already closed.
DOC: Issues around close/1 and stream pairs.
MODIFIED: Allow stream_pair(+,-,-) with a stream argument and do not bind already closed streams
Updated http