Add streamy PROPFIND responses.

Backport revs 1.94, 1.95, and 1.96 of mod_dav.c, and 1.67 of mod_dav.h

Submitted by: Ben Collins-Sussman <sussman@collab.net>
Reviewed by: gstein, striker, jerenkrantz


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/APACHE_2_0_BRANCH@100361 13f79535-47bb-0310-9956-ffa450edef68