fsrvp: add fss_agent rpc server

fsrvp: add fss_agent rpc server

The Samba fss_agent rpc server is an implementation the the File Server
Remote VSS (Volume Shadow Copy) Protocol, or FSRVP for short.

FSRVP is new to Windows Server 2012, and allows authenticated clients to
remotely request the creation, exposure and deletion of share snapshots.

The fss_agent rpc server processes requests on the FssAgentRpc named
pipe, and dispatches relavent snapshot creation and deletion requests
through to the VFS asynchronously.
The registry smb.conf backend is used to expose snapshot shares, with
configuration parameters cloned from the base share.

There are three FSRVP client implementations that I'm aware of:
- Samba rpcclient includes fss_X commands.
- Windows Server 2012 includes diskshadow.exe.
- System Center 2012.