Unionmount: proxying the control port

[Sergiu Ivanov]

Hello,

It was agreed that unionmount should forward some of the RPCs invoked
on its control port to the mountee.  Most (if not all) of such RPCs
are the fsys_* ones.  I've made up a list of RPCs which should be
proxied in my opinion and I've also added a short explanation as to
how this proxying should work:

* fsys_goaway: Both the mountee and unionmount should go away.

* fsys_syncfs: Sync both the mountee and the underlying filesystem.

* fsys_set_options: This RPC should be forwarded to the mountee
  completely.  unionmount does not have any command line switches that
  would make much sense being altered at run-time.

* fsys_get_options: This RPC should be forwarded to the mountee
  completely.  The reasoning is the same as for fsys_set_options.

There also are some RPCs which I am not certain about:

* fsys_getfile: I don't really understand what this one does.

* fsys_forward: From what antrik told me, this RPC should be forwarded
  to the mountee completely; however, antrik also told me that this
  RPC could be ignored, so I think I'll try to forward this completely
  and see if it works.

In any case, I think that the fact that these RPCs are so rarely used
shows that unionmount can forward them completely to the mountee
without bothering to handle them specially.

Regards,
scolobb