Add a new group definition to sys/conf.h: GID_TTY.

Our current TTY layer uses a set-uid application called ptchown to
change ownership of a PTY slave device. The new TTY layer implements
this functionality through a new ioctl().

By accident I discovered Darwin's TTY layer also uses this approach.
Because of this, they also have a GID_TTY.

Approved by: philip (mentor)