Fix umask handling when creating new files

  * src/transform.c (transform_save): faulty umask arithmetic would cause
    overly-open file modes when the umask contains "7", as the umask was
    incorrectly subtracted from the target file mode

Fixes CVE-2013-6412, RHBZ#1034261

https://bugzilla.redhat.com/show_bug.cgi?id=1034261 for reference.