configurable timestamp logging, patch included

[mcosby]

This patch adds configurable timestamp logging to lighttpd's mod_accesslog, in the same format as apache's %{format_string}t option.

It keeps the time caching fully intact but adds a new restriction that you can't use %t twice in the same access log specification, since there's only one slot in the cache. I doubt that restriction will impact any current users (there is no point in using %t twice when it's not configurable) and that the vast majority of future users will never notice the restriction.

[mcosby]

patch

[glen]

please document the config changes in wiki and doc/accesslog.txt