ws: Add cockpit_web_response_abort() to stop streaming response

When we are streaming a web response, and half-way an error occurs
and we cannot get the remaining data to stream, we need to terminate
the web response.

cockpit_web_response_abort() does this by just disconnecting the
web response. The caller should be able to tell whether the data
is incomplete based on length header or chunking.
ws: cockpit_web_response_get_state() and expose web response state

Allow others to see what state a web response is currently in.
This helps prevent callers from having to carry around the identical
state.
ws: Strip query string when handing HTTP requests

We never use a query string, and some callers like jQuery add one
automatically in certain cases.
ws: Change how CockpitWebServer handle-resource signal detail works

To match subtrees, connect using detail handle-resource::/component/, to
match actual to level paths use detail handle-resource::/component
ws: Expose resources from remote hosts via HTTP

The resources are under the /cache and /res paths. See doc/urls.md
for more information.

We stream these from cockpit-agent using channels with payload
type resource1.

We also track when the web service requests checksum and module
listing information, and keep track of which host has which checksums
so we can send requests appropriately later.
ws: Serve the / HTTP path with its own handler

Since the response to / cannot be aggressively cached, unlike almost
everything else ... include dynamic content right in the response to
minimize server round trips.

Removed the unused avatar data url. If we do want an avatar on
the login page, it should be retrieved as a normal image and have
its own handler.
ws: Handle favicon.ico and apple-touch-icon.png separately

These don't get handled via /static or the later resource1
payload channel. These are special cases.
ws: Transfer login, module, host info using environment block

This is a JSON block called the environment which is used to pass the
necessary information to start up cockpit. It is received by the
javascript.

When not yet logged in, the environment contains very little
information, once logged in it is fully populated.

The environment is seeded in the '/' index handler, to avoid multiple
round trips. If a login occurs, then the full environment is transferred
as a response to the '/login' request.
static: Separate the login and boot code from shell module

We use the environment to load the cockpit-web as a single module
called 'shell'. This uses a simple module code, which is not
expected to be used to used with the later module loading.

In order to test this particular code you'll need to run:

ln -s /path/to/cockpit/src/web ~/.local/share/cockpit/shell
shell: Move cockpit-web into it a module called 'shell'

And install it to the appropriate location.
ws: Remove http roots option for cockpit-ws

This is no longer used or relevant. Our HTTP resources come
from either the static directory or modules
test: Update tests for new module loading