When exporting request headers to HTTP_* environment variables, drop variables

whose names contain invalid characters. Describe in the docs how to restore the
old behaviour.

Submitted by: Malte S. Stretz <mss apache org>


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1053353 13f79535-47bb-0310-9956-ffa450edef68