When the "dynamic" ENGINE loads another ENGINE from a shared-library, it

When the "dynamic" ENGINE loads another ENGINE from a shared-library, it
essentially overwrites itself with the new ENGINE, with the exception of
reference counts, ex_data structures, and other 'admin' elements. However
if the new ENGINE doesn't populate certain elements, there's the risk of
the "dynamic" ENGINE's elements showing through - the "cmd_defns" were just
one of the possibilities. This implements a more comprehensive cleanup.