Prevent a case of SSI timefmt-smashing with filter chains including

Prevent a case of SSI timefmt-smashing with filter chains including
multiple INCLUDES filters:

* modules/filters/mod_include.c (add_include_vars): Drop unused
  timefmt argument.
  (add_include_vars_lazy): Take timefmt argument.
  (get_include_var, handle_printenv): Pass time format from context.

PR: 39369

Backport of r757376 from trunk resp. r773352 from 2.2.x.

Submitted by: jorton
Backported by: rjung
Reviewed by: wrowe, humbedooh

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.0.x@1469722 13f79535-47bb-0310-9956-ffa450edef68