slcanpty: Add support for the Unix 98 pseudo-terminal interface

slcanpty: Add support for the Unix 98 pseudo-terminal interface

Most Linux distributions do not configure their kernels to use the BSD
pseudo-terminal interface (/dev/pty* and /dev/tty*) anymore; they uses the
Unix 98 pseudo-terminal interface instead (/dev/ptmx and /dev/pts/*).

http://www.kernel.org/doc/man-pages/online/pages/man4/pts.4.html

This change follows the slcan_attach changes:
http://sourceforge.net/tracker/index.php?func=detail&aid=3467521&group_id=146269&atid=764681

Signed-off-by: Ulrich Escher<git@myvdr.de>
Signed-off-by: Oliver Hartkopp <socketcan@hartkopp.net>