Conflate Qnil and Qunbound for `symbol-function'.

Conflate Qnil and Qunbound for `symbol-function'.
* src/alloc.c (Fmake_symbol): Initialize `function' to Qnil.
* src/lread.c (init_obarray): Set `function' fields to Qnil.
* src/eval.c (Fcommandp): Ignore Qunbound.
(Fautoload, eval_sub, Fapply, Ffuncall, Fmacroexpand):
* src/data.c (Ffset, Ffboundp, indirect_function, Findirect_function):
Test NILP rather than Qunbound.
(Ffmakunbound): Set to Qnil.
(Fsymbol_function): Never signal an error.
(Finteractive_form): Ignore Qunbound.