Add a new #define to disable pty_pre_init.

Add a new #define to disable pty_pre_init.

OS X dislikes us calling the setuid or setgid syscalls when not
privileged, even if we try to set ourselves to the _same_ uid/gid.
Since I don't anticipate this code needing to run setuid on OS X, and
since I do anticipate wanting to handle multiple ptys in a single
process so that pty_pre_init would be useless anyway, the simplest fix
seems to me to be just conditioning out the whole of pty_pre_init
completely.