setconfig: fix crash on dynamic multi-value plugin options

[wqxoxo]

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