Add microsecond timestamps to the error log.

The new function ap_recent_ctime_ex allows for
optional formatting hints. It checks the provided buffer
length and returns the number of consumed bytes. This is necessary,
because when using options, the length of the formatted time string
is no longer constant.

The only option implemented currently is the addition of microsecond
fractions to the timestamp.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@952203 13f79535-47bb-0310-9956-ffa450edef68