Handle `kd == NULL` gracefully with kvm_close/kvm_geterr
Don't segfault (kvm_close) or "just work by accident" (kvm_geterr).
- kvm_close: return -1/set errno == EINVAL.
- kvm_geterr: return NULL.
Document the new error condition in kvm_close(3).
MFC after: 1 week
Sponsored by: Dell EMC Isilon