DEFINE-FUNCTION-NAME-SYNTAX for CAS is not strict enough

[Douglas Katzman]

No check is made that the second element of the name is itself a valid name, leading to:

* (fboundp '(sb-ext:cas (foo)))
=> NIL

* (fboundp '(sb-ext:cas "garbage"))
=> NIL

Ideally it would signal the same condition as does this example:
* (fboundp '(setf (foo)))

debugger invoked on a SIMPLE-TYPE-ERROR: invalid function name: (SETF (FOO))
restarts (invokable by number or by possibly-abbreviated name):
  0: [ABORT] Exit debugger, returning to top level.

(SB-INT:LEGAL-FUN-NAME-OR-TYPE-ERROR (SETF (FOO)))