CLEANUP: tcpcheck: mention that we're a bit far for a sync errno

CLEANUP: tcpcheck: mention that we're a bit far for a sync errno

The collection of errno in tcpcheck_eval_connect() and tcpcheck_main()
is quite far from the production location, and the risk of having a
zero errno is definitely not null. Tests show that this works, so
better not try to fix something not broken, but at least place a
comment there indicating that it's not necessarily super-reliable.
This would need to be revisited the day we finally store errno in
the connection.