FIX: wrong function types in ufunc_object.c

assign_reduce_identity_{zero,one} were treated as
PyArray_AssignReduceIdentityFunc, but that type has
an extra void* argument. Added the argument.
FIX: remove unused variables and add casts

Makes the build complete with fewer warnings.
MAINT: ignore some more generated C files.
Merge pull request #3653 from larsmans/c-fixes

FIX: call through wrong function pointer type + minor stuff