Fix handling of unknown state tokens in If headers:

* modules/dav/main/mod_dav.h: Add dav_if_unknown to dav_if_state_type
enum.

* modules/dav/main/util.c (dav_add_if_state): Set returned type to
dav_if_unknown for an unknown state token.
(dav_validate_resource_state): Evaluate dav_if_unknown to fail
match unless in a Not condition.

PR: 37288


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@329562 13f79535-47bb-0310-9956-ffa450edef68