ws: Don't complain if SSH connection already close when closing

When closing an SSH connection, don't complain if it's already
closed. libssh returns SSH_REQUEST_DENIED in this case.
ws: Send a 'channel-seed' in the WebSocket init message

This allows various web sockets to generate unique channels, without
having to get into NAT'ing everything in CockpitWebService.

This is necessary once various frames are the source of channels.
They'll need to be seeded appropriately so that they pick unique
channel ids.

The way that channels get their id, also needs to change a bit.
The id can't be allocated until the transport is open. So queueing
of messages happens at the channel level now.
ws: Send user info in the 'init' control channel message

Rather than filling in the user into the index page environment,
send it in the 'init' control message, along with the cockpit
setup info.

The index page will be changing, and this is much cleaner.

Closes #1332
ws: Show a login page for any .html resource being accessed

For other resources besides the main page, show a login page if user
doesn't have a cookie.

One big difference, we now refresh the page after completing a
sucessful login.
static: Change the styles of login page when shown inline

When not logging into cockpit as a whole, just show a basic login
screen on a white background. This will help not confuse users as
to what they're logging into.