Implement a runtime test that checks whether it's possible to use poll() to

Implement a runtime test that checks whether it's possible to use poll() to
read data from a pty device. The test only runs the first time that the
can_poll_pty() function is called (currently happens when the first
PollScheduler instance is created). The test is known to fail on Darwin and
we're not sure how many other kernels might have a similar problem, this
runtime test allows us to avoid having to maintain special cases for various
kernels.

svn path=/main/trunk/; revision=11025