Webtiles: support for games.conf.d/

Add support for games_conf_d option, which sets a dir where additional
*.toml files specifying game configuration will be loaded from. Issues
with this initial support:

* Debug logging is hackily implemented through another env var (long
  term solution: allow passing logging parameters to server.py argv)
* Poor error handling for unexpected content in the additional toml
  files.
* Code to load additional config is very complex (hopefully mostly due
  to extensive error handling...)
Webtiles: chown pidfile when running as non-root

Without this, you can't delete the pidfile when running as a non-root
user.
Webtiles: Silence spurious error during webserver shutdown.

This happened to me on OSX with no games specified, I'm not sure what
the root cause is but seems harmless.
Webtiles: Add example for "[[games]].options" in config.toml.sample
Webtiles: config.dgl_status_file improvements.

* Make it optional
* Check it's writable on startup
Webtiles: Debug logging for listen socket setup
Webtiles: Add config.enable_access_log
Webtiles: More config.toml.sample documentation
Webtiles: cross-site protection should ignore protocol
Webtiles: Simplify devteam_file load logic
Webtiles: Add startup check for config.password_db

Warn if we're going to create it, and fail early if we won't be able to.
Webtiles: init_player_program improvements

* Make it optional
* End-user message on failure should be more generic
Webtiles: move init_logging to conf.py so we can log earlier
Webtiles: get rid of 'from util import *'
Webtiles: Add some debug logging to check_config