Allow retries when checking TCP high-water stats

Allow retries when checking TCP high-water stats

In the TCP high-water checks, "rndc stats" is run after ans6 reports
that it opened the requested number of TCP connections.  However, we
fail to account for the fact that ns5 might not yet have called accept()
for these connections, in which case the counts output by "rndc stats"
will be off.  To prevent intermittent "tcp" system test failures, allow
the relevant connection count checks to be retried (just once, after one
second, as that should be enough for any system to accept() a dozen TCP
connections under any circumstances).