ws: Proper errors returned from CockpitWebServer static files

Only return 404 for files not found, and don't make a big deal about
this in the logs. Handle 403 appropriately, in addition for denying
listing directories. 500 for everything else.

Don't leak internal file lookup errors, which could be a security
breach.
ws: Make CockpitWebServer serve static files from multiple directories

This is really handy during debugging.
web: Include <script> tags for js files in index.html when in debug mode

When in --enable-debug mode, include <script> tags for each of the
javascript files individually in index.html so we can easily see
the relevant line of an exception or other failure.
ws: Allow overriding where cockpit-agent is when in debug mode

In debug mode we'd like to run out of $builddir. So allow an additional
--agent-path agrument to cockpit-ws (only valid when debug build)
which allows us to override the default path.
web: Clearer symmetrical debugging in channel.js

Debug both in and out messages the same for clarity.