frr: default permission issue eases information leaks

An issue was discovered in FRRouting FRR (aka Free Range Routing) through 7.3.1. When using the split-config feature, the init script creates an empty config file with world-readable default permissions, leading to a possible information leak via tools/frr.in and tools/frrcommon.sh.in. NOTE: some parties consider this user error, not a vulnerability, because the permissions are under the control of the user before any sensitive information is present in the file