Improve the logic in mod_dav_svn's implementation of LOCK.

* subversion/mod_dav_svn/lock.c
  (append_locks): Only allow locks on paths that are in the repository,
    disallow anonymous locks earlier, move the repos variable to function
    scope and cleanup some messy uses of resource->info->repos.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1455352 13f79535-47bb-0310-9956-ffa450edef68