mod_log_config: instead of using the new dedicated

pattern format "%M" for duration milliseconds,
overload the existing "%D" to choose the time precision
("%{s}D" for seconds, "%{ms}D" for milliseconds and
"%{us}D" for microseconds).

The existing %T and %D without precision are kept for
compatibility.

The previously introduced "%M" (r1677187) is removed,
it has not yet been released. Format pattern characters
are rare, so we should only use a new one if an
existing one isn't a good fit.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1680895 13f79535-47bb-0310-9956-ffa450edef68