There are two related changes here:
1. Also check for S_ISCHR/FILE_TYPE_CHAR when checking for pipes, so
that we detect ttys as well, which are also not seekable.
2. Always set position=-1 (i.e. ftell will return false) when a pipe
is detected. Previously position=0 was sometimes used, depending on
whether we're on Windows/Linux and whether the FD or FILE codepath
was used.
