We had an assert(!(ot->type & OPT_MULTI)) which crashed when using
setconfig on a plugin option marked as both dynamic and multi.
The fix changes plugin_set_dynamic_opt to accept an array of values
(scalar options pass a 1-element array, multi options pass the complete
set). For multi options, setconfig replaces ALL values atomically - an
empty array clears them.
Fixes: #8295
Changelog-Fixed: setconfig no longer crashes on dynamic multi-value plugin options
