Accept "kw=1" parameter in mod_dav_svn GET query string as a flag
which means "perform keyword substitution on the returned file content
stream".

* subversion/mod_dav_svn/dav_svn.h
  (struct dav_resource_private): add keyword_subst flag.

* subversion/mod_dav_svn/repos.c
  (svn_subst.h): Add #include.
  (parse_querystring): Set keyword_subst flag if query string contains
    "kw" parameter with value "1", and preserve that flag's state when
    redirecting.
  (set_headers): Don't send "Content-length" header when doing keyword
    substitution as we can't know the size of the resulting stream.
  (deliver): Perform keyword substitution on the returned stream if
    requested.

Patch by: jinfroster <jinfroster{_AT_}mail.ru>
          (Tweaked by me.)


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1466055 13f79535-47bb-0310-9956-ffa450edef68