services: configuration: Rewrite 'alist?' procedure.

* gnu/services/configuration.scm (alist?): Rewrite.

Co-authored-by: Ludovic Courtès <ludo@gnu.org>

gnu/services/configuration.scm

@@ -436,7 +436,11 @@ the list result in @code{#t} when applying PRED? on them."
 (define list-of-strings?
   (list-of string?))
 
-(define alist? list?)
+(define alist?
+  (match-lambda
+    (() #t)
+    ((head . tail) (and (pair? head) (alist? tail)))
+    (_ #f)))
 
 (define serialize-file-like empty-serializer)