* src/estream.c (estream_internal): Add field SAMETHREAD.
(init_stream_lock, lock_stream, trylock_stream, unlock_stream): Use it.
(parse_mode): Add arg SAMETHREAD and parse that keyword.
(es_initialize): Rename to ...
(init_stream_obj): this.  Add arg SAMETHREAD.
(es_create): Add arg SAMETHREAD.  Call init_stream_lock after
init_stream_obj.
(doreadline): Call es_create with samethread flag.
(es_fopen, es_mopen, es_fopenmem, es_fopencookie, do_fdopen)
(do_fpopen, do_w32open): Implement "samethread" keyword.
(es_freopen): Take samthread flag from old stream.
(es_tmpfile): Call es)_create w/o samethread.