Fixes #15870: Deserialization issues caused by mc_config_get_string()

A simple fix to avoid php errors when a client app requests a config option that has an array value.  Some config options are always arrays others can be a threshold vs. an array.  With this change, we will avoid the php error and return a string that is easy parse.

This fix is proposed for both master-1.2.x and master.  In master, we can add new API that returns structured data.  And ones that reduce the need to retrieve configs to simulate web app behaviors.

See the associated issue for discussions we had sometime back.