tests: kqueue: improve diagnostics for our CPONFORK test

Notably, confirm in the child that our close-on-fork fd is actually
closed, and break RECV_ALL out into a table and check each bit
individually to provide a better message when the test fails.

While we're here, just switch to waitid() rather than trying to identify
the point where we have to make the switch. This reduces maintenance
slightly, as keeping our assertion static would require still adding to
a _RECV_ALL mask *just* for that purpose.

Reviewed by: kib, markj (both slightly earlier version)
Differential Revision: https://reviews.freebsd.org/D56372