split ztree find_child into read/writer variants

The internal branching in the unified version of the function
seemed like it might potentially be dangerous for optimization
with regard to rcu_dereference, and probably poorly-optimized in
general for both cases due to the loop-internal branching.
Change compiled test extension to .t

... because this might make life easier for some invocations
and/or versions of "prove"
regression suite resiliency stuff

Apparently under "prove", Test::More::BAIL_OUT can skip over END
blocks somehow (at least, that's the behavior I seem to be
getting), so replace that with a simple "die" (which is not the
same, but the result is better).

Also, the 001basic checks for specific testsuite modules were
outdated vs the list in _GDT.pm and redundant anyways.
djbdns: use only the mtime seconds for auto serial
Bugfix: backwards retval in statio_check_socks