General cleanup:

* Change version number to 2.3
* Change documentation links to new layout
* Updated, concise but hopefully correct discussion of build system
  behaviour w.r.t. threads on FreeBSD
* A little more discussion of buildconf and minimum libtool version on
  Darwin
* More caveat on availability of Windows sourceballs and installers

Initially submitted by pgolluci, subsequently kicked around some.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@359993 13f79535-47bb-0310-9956-ffa450edef68