rc.d/ggated: Simplify service script

• Use the standard *_flags variable for additional flags.
• Style: do not create unnecessary variables
• Do not set the defaults in the service script. This is what /etc/defaults/rc.conf is for.
• Do not set additional flags via commands_args. ggated_flags are already included in the final invocation. See rc.subr(8) for details.
• Document the meaning of ggated_config in /etc/defaults/rc.conf.

Approved by: eugen (src)
Fixes: c068632981eb Add ggated rc script
Differential Revision: https://reviews.freebsd.org/D34439