setf: read-modify-write macros should deal with multi-value places gracefully.

setf: read-modify-write macros should deal with multi-value places gracefully.

  * In PUSH, PUSHNEW, POP, REMF, INCF, DECF and DEFINE-MODIFY-MACRO the
setf-expansion was being obtained via GET-SETF-METHOD, which is the
CLtL1 version of GET-SETF-EXPANSION, but throws an error if a PLACE has
multiple values.  This also pre-dates the adoption of VALUES places.

  * The most reasonable interpretation of the spec appears to be that
any values after the first are to be ignored upon reading and set to NIL
upon writing.

  * To do so, change each use to SB!XC:GET-SETF-EXPANSION instead
of GET-SETF-METHOD, and bind any symbols in the list of new value
locations to NIL before invoking the setter form.