structure, in preparation for wanting more than one of them in a
single process. This can't be done cleanly, because the whole
business with pty_pre_init pre-allocating the pty rather assumes we
want a known number of the things before we drop privileges; so
there's a horrid hack to make pty_pre_init work on platforms that
have at most one pty instance per process, but at the same time
things ought to work sensibly with more than one per process _if_
pty_pre_init isn't required.
[originally from svn r5261]
/dev/ptyXX we can open: we must also check that we can open and use
the corresponding /dev/ttyXX, because if it's been left in the wrong
mode then we will look terribly silly when we fork and _then_
discover our pty is unusable.
[originally from svn r5257]
