net: dialChannel should not treat an expired deadline as noDeadline.

net: dialChannel should not treat an expired deadline as noDeadline.

Now, only a zero deadline is interpreted as noDeadline.  Any other time
in the past yields an immediate timeout.

TestConnectDeadlineInThePast already covers this case.  We just need to
un-skip it for plan9, where dialChannel is used.

Change-Id: I995fd1a632c31f8004dac772c3d7c43a2a5853b0
Reviewed-on: https://go-review.googlesource.com/8435
Reviewed-by: Mikio Hara <mikioh.mikioh@gmail.com>