the stream position was at offset 0.
This corrects that assumption by querying the stream for it's position
when it detects the 'a' "flag" in the mode parameter to fopen.
Also added a test for plain files and amended the userstreams test to
take this into account.
I've moved EOF detection into the streams layer; a stream reader
implementation should set stream->eof when it detects EOF.
Fixed test for user streams - it still fails but that is due to an output
buffering bug.