core: Make sure and check that LimitXMLRequestBody fits in system memory.

core: Make sure and check that LimitXMLRequestBody fits in system memory.

LimitXMLRequestBody can not exceed the size needed to ap_escape_html2() the
body without failing to allocate memory, so enforce this at load time based
on APR_SIZE_MAX, and make sure that ap_escape_html2() is within the bounds.

Document the limits for LimitXMLRequestBody in our docs.

Merge r1898686 from trunk.
Submitted by: ylavic, rpluem
Reviewed by: ylavic, covener, rpluem

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1898693 13f79535-47bb-0310-9956-ffa450edef68