FIX: locally uploaded files are world-writable
add `GetUmask()` to util.h
apply umask to 0666 to obtain backend file mode
guard access of umask with global lock
add documentation for GetUmask()
FIX: assert successful mutex locking
FIX: use internal `Move()` method in LocalUploader
TEST: integration test for usage of umask
disable test 571 on S3 (not applicable)
Merge pull request #573 from reneme/fix-backend_permissions

FIX: Locally Uploaded Files are World-Writable