RestoreGUCState applied InitializeOneGUCOption to already-live
GUC entries, causing any malloc'd subsidiary data to be forgotten.
We do want the effect of resetting the GUC to its compiled-in
default, and InitializeOneGUCOption seems like the best way to do
that, so add code to free any existing subsidiary data beforehand.

The interaction between can_skip_gucvar, SerializeGUCState, and
RestoreGUCState is way more subtle than their opaque comments
would suggest to an unwary reader.  Rewrite and enlarge the
comments to try to make it clearer what's happening.

Remove a long-obsolete assertion in read_nondefault_variables: the
behavior of set_config_option hasn't depended on IsInitProcessingMode
since f5d9698a installed a better way of controlling it.

Although this is fixing a clear memory leak, the leak is quite unlikely
to involve any large amount of data, and it can only happen once in the
lifetime of a worker process.  So it seems unnecessary to take any
risk of back-patching.

Discussion: https://postgr.es/m/4105247.1616174862@sss.pgh.pa.us