function-lambda-expression: Return the name of a generic function.

The third value of
(function-lambda-expression (defgeneric foo ()))
is now FOO, and not
(LAMBDA (&REST SB-PCL::ARGS) :IN SB-PCL::MAKE-INITIAL-DFUN)
Reported by Marco Baringer.
Stop (describe (make-instance 'generic-function)) from crashing.

All the describy things can be pried out of a generic function only if
it's a standard-generic-function.
Also add an FNDB entry for FUNCTION-LAMBDA-EXPRESSION.
Add some missing FNDB declarations.