FallbackResource: Support for the 'disabled' argument

Backport of r1398066 from trunk resp. r1416585 form 2.4.x.

Submitted by: gryzor
Reviewed by: wrowe, rjung
Backported by: gryzor


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.2.x@1445133 13f79535-47bb-0310-9956-ffa450edef68