Make unixctl_command_register() idempotent

Make unixctl_command_register() idempotent

The assert() statement in unixctl_command_register() implies that it is
intended to be idempotent but inserting the same name and callback twice
would fail because:

* The callback is not stored directly in the hash, rather it the
  cb element of a struct unixctl_command which is stored in the hash.

* Insertion would be attempted even if the entry was already present.