Check permissions on startup

[Kevin Zheng]

SSHGuard should check its ability to block and release addresses, whether using one of the firewall backends or modifying files. Most of these operations require root, however, it would be silly to require root permissions to use the "null" backend.

Ideally, when SSHGuard detects that it has insufficient privileges, it should fail as loudly as possible.

Reported by: Libertas [e-mail]

[Kevin Zheng]

Fixed by switch to sshg-fw in d1a7a91.