net-mgmt/semaphore: fix rc script config path

The rc script defaulted to semaphore_config.json, but the port installs
config.json, so the service did not start on a fresh installation.

Installations that created semaphore_config.json by hand can keep it via
semaphore_config in rc.conf.

Reported by: Niclas Wagner (via private mail)
Approved by: portmgr (runtime fix blanket)
Sponsored by: Netzkommune GmbH