block pthread cancellation in openpty function

being a nonstandard function, this isn't strictly necessary, but it's
inexpensive and avoids unpleasant surprises. eventually I would like
all functions in libc to be safe against cancellation, either ignoring
it or acting on it cleanly.

don't write openpty results until success is determined

not only is this semantically more correct; it also reduces code size
slightly by eliminating the need for the compiler to assume the
possibility of aliasing.

incorrect check for open failure in openpty function

-1, not 0, indicates failure

initial check-in, version 0.5.0