When `fctx_query()` is called, a DTrace probe (if enabled) prints the
fetch context address, the upstream server address and port, and the
latest known SRTT for the server.
Introduces a top-level dtrace/ directory for user-contributed trace
scripts that consume the USDT probes exported by libdns, libns, and
libisc. Ships with delegdb-trace.stp, which streams every insertion,
eviction, and rndc flush-delegation removal in the delegation cache,
and a README pointing at the provider files and explaining how to list
and run the probes on Linux (SystemTap) and on FreeBSD/macOS (DTrace).
