mod_dav, mod_dav_fs: Add opt-in support for controlling resource

mod_dav, mod_dav_fs: Add opt-in support for controlling resource
modification times via the X-Oc-Mtime header in the PUT and MKCOL
method implementations.

* modules/dav/fs/mod_dav.h: Extend dav_hooks_repository struct
  with set_mtime function pointer.

* modules/dav/fs/repos.c (dav_fs_set_mtime): New function.

* modules/dav/main/mod_dav.c (dav_cmd_davhonormtimeheader,
  dav_parse_mtime): New functions.
  (dav_method_put, dav_method_mkcol): Add X-Oc-Mtime handling.
  (dav_cmds): Add DAVHonorMtimeHeader directive.

Submitted by: Leo <i hardrain980.com>
Github: closes #556

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1929581 13f79535-47bb-0310-9956-ffa450edef68