*) Set content type and content length.
*) Add the option to explicitly set the base directory against which
wildcards are interpreted.
*) Make sure that we set an explicit handler in the fixups hook.
*) Add the RRDGraphFormat option to specify the desired format for a
graph, overriding the file extension.
*) Allow the DEF element to support an expression as a path, so that RRD
files can be mapped dynamically from the original request URL using
expressions.