games/ioquake3: add rc file and example server.cfg file

The port games/ioquake3-server can be used to build a dedicated
server for Quake 3 and derivative games. However, it doesn't come
with an rc script to launch at system boot or on demand. This makes
it a bit tricky to run the dedicated server.

ioquake3-server is a "slave port" that depends on the master port
games/ioquake3 to actually retrieve the source and build everything.

This patch adds an rc script plus example server.cfg to the master port,
which is installed when the slave port is installed.

PR: 239180
Submitted by: kennedy.greg@gmail.com