setf: pre-defined setf-expanders should handle multiple value places gracefully.

setf: pre-defined setf-expanders should handle multiple value places gracefully.

  * The GETF, LOGBITP, LDB and MASK-FIELD setf-expanders all take a
PLACE argument, the setf-expansion for which was being obtained via
GET-SETF-METHOD, which is the CLtL1 version of GET-SETF-EXPANSION, but
throws an error if a PLACE 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.