mu-sexp: tighten definition of plistp

In a plist, we also require the first of each two element pairs to start
with ':'

Fixes #2830.

lib/utils/mu-sexp.cc

@@ -326,7 +326,9 @@ Sexp::plistp(Sexp::const_iterator b, Sexp::const_iterator e) const
 	else if (b + 1 == e)
 		return false;
 	else
-		return b->symbolp() && plistp(b + 2, e);
+		return b->symbolp() &&
+			b->symbol().name.starts_with(':')
+			&& plistp(b + 2, e);
 }