1.0.28.3: ABOUT-TO-MODIFY-SYMBOL-VALUE doesn't choke on FUNCTION subtypes

1.0.28.3: ABOUT-TO-MODIFY-SYMBOL-VALUE doesn't choke on FUNCTION subtypes

 * Evaluating eg. a SET when the type of the variable as been
   proclaimed to be a subtype of FUNCTION used to break, since
   ABOUT-TO-MODIFY-SYMBOL-VALUE uses %%TYPEP to check the type, and
   function subtypes are not normally acceptable type specifiers to
   TYPEP.

   SBCL is, however, able to reason about such types, so we add an
   optional STRICT argument to %%TYPEP defaulting to T, which
   A-T-M-S-V give as NIL to allow checking of function subtypes.

   Reported by Lorenz Mösenlechner.