Rework DSO error reporting to be more flexible & informative.

This patch covers os/2, unix & win32. Other platforms still need some
adjustment (BeOS, AIX).

Reviewed by:	rbb, gstein


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@85357 13f79535-47bb-0310-9956-ffa450edef68