bug#68931: 30.0.50; Gnus byte-compilation error with (display . [not expire]) with git emacs

[Mattias Engdegård]

>> Debugger entered--returning value: "Malformed function ‘#[0 
>> \"\\301\\300!\\207\" [expire gnus-article-marked-p] 2]’"
...
>> * gnus-summary-display-make-predicate((not expire))

Apparently, gnus-category-make-function-1 creates code that isn't really valid 
Lisp but that we have previously allowed anyway: (F ...) where F is a 
(non-symbol) function value, instead of using `funcall`.

We could (and probably should) allow this for compatibility but perhaps it's 
time to at least start warning about it? It makes the quirky semantics of a 
Lisp-2 even quirkier.