Minor rework of ALTER TABLE SET RelOptions code

While working with my New Options Engine patch
https://commitfest.postgresql.org/patch/4688/
I found out that I can detach a small portion of it as a separate
patch. 
It has own value, even if big patch is never committed, and it would make 
smoother further committing of big patch if we ever get to it.

Patch description is following:

1. `isnull` variable is actually needed in a very narrow scope, so              
it is better to keep it in that scope, not keeping it in mind in while          
dealing with the rest of the code.                                              
                                                                                
2. Toast table RelOptions are never altered directly with ALTER command.        
One should do ATLER to a heap relation and use toast. reloption namespace       
to address toast's reloption. If you get `ATExecSetRelOptions` called with      
`RELKIND_TOASTVALUE` relation in the args, something is really wrong. We        
should throw asserts, errors and whistle as loud as we can.

-- 
Nikolay Shaplov aka Nataraj
Fuzzing Engineer at Postgres Professional
Matrix IM: @dhyan:nataraj.su