Log Message:
vte3: improve patch to work around NetBSD posix_openpt(3) defect

Now that as of roughly 9.99.101 we have a posix_openpt(3) that applies
O_NONBLOCK and O_CLOEXEC, we don't need the fallback code. Now we
dynamically detect whether we need the fallback using the best
approximation we can (an assumption backed by a test case I recently
added). Tested on 9.3_STABLE, 10.0_BETA, and 10.99.4.