Refactor: Remove global booth_conf variable from network functions.

• Add braces around some nearby conditional blocks.

• Make add_hmac a static function and move it up in the source file to avoid a function declaration.

• Convert _FOREACH_NODE to FOREACH_NODE in booth_udp_broadcast_auth.

• Change other network functions to be static.

Co-authored-by: Jan Pokorny <jpokorny@redhat.com>