api2/nodes: add termproxy and vncwebsocket api calls

api2/nodes: add termproxy and vncwebsocket api calls

Even though it has nothing to do with vnc, we keep the name of the api
call for compatibility with our xtermjs client.

termproxy:
verifies that the user is allowed to open a console and starts
termproxy with the correct parameters

starts a TcpListener on "localhost:0" so that the kernel decides the
port (instead of trying to rerserving like in pve). Then it
leaves the fd open for termproxy and gives the number as port
and tells it via '--port-as-fd' that it should interpret this
as an open fd

the vncwebsocket api call checks the 'vncticket' (name for compatibility)
and connects the remote side (after an Upgrade) with a local TcpStream
connecting to the port given via WebSocket from the proxmox crate

to make sure that only the client can connect that called termproxy and
no one can connect to an arbitrary port on the host we have to include
the port in the ticket data

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>