mod_dav: Fix PR 55306.

Makes mod_dav no longer require that the lock token be provided when the
source of a COPY is locked.  The prior behavior was in violating of
RFC 4918 which says that the lock token is only required on resources
that may be modified by the method.

* modules/dav/main/mod_dav.h
  (DAV_VALIDATE_NO_MODIFY): New flag to be passed to dav_validate_* functions.

* modules/dav/main/mod_dav.c
  (dav_method_copymove): Use the new flag when calling dav_validate_request()
    on the COPY source.

* modules/dav/main/util.c
  (dav_validate_resource_state): Use the flag to decide to ignore if the lock
    token is not provided.

Submitted by: breser
Reviewed/backported by: jim


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1533447 13f79535-47bb-0310-9956-ffa450edef68