Add a helper shell function, rndc_dumpdb(), which provides a convenient
way to call "rndc dumpdb" for a given server with optional additional
arguments.  Since database dumping is an asynchronous process, the
function waits until the dump is complete before returning, which
prevents false positives in system tests caused by inspecting the dump
before its preparation is finished.  The function also renames the dump
file before returning so that it does not get overwritten by subsequent
calls; this retains forensic data in case of an unexpected test failure.

(cherry picked from commit ab78e350)