core, mod_cache: Ensure RFC2616 compliance in ap_meets_conditions()

with weak validation combined with If-Range and Range headers. Break
out explicit conditional header checks to be useable elsewhere in the
server. Ensure weak validation RFC compliance in the byteranges filter.
Ensure RFC validation compliance when serving cached entities. PR 16142

trunk patch: http://svn.apache.org/r1479905
2.4.x patch: http://people.apache.org/~minfrin/httpd-if-range-fix2.4.patch

Submitted by: minfrin
Reviewed by: jim, wrowe


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1487127 13f79535-47bb-0310-9956-ffa450edef68