mod_rewrite: Add an opt-in RewriteOption to control
merging of RewriteBase. Merging was off until 2.2.22
and turned on unconditionally in 2.2.23.
PR 53963.

Submitted by: covener
Backport by: rjung
Reviewed by: wrowe, covener

Merge of r1410681 and r1447426 from trunk resp.
r1418954 and r1447448 from 2.4.x.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.2.x@1447508 13f79535-47bb-0310-9956-ffa450edef68