PR#4012: rmtree: use fork
    
    Merges #4012
    https://pagure.io/koji/pull-request/4012
    
    Fixes: #3755
    https://pagure.io/koji/issue/3755
    koji-gc occasionally gets Directory not empty fault on delete
    
    Fixes: #2481
    https://pagure.io/koji/issue/2481
    koji.util.rmtree() is not multi-process/thread safe