rc.conf.5: modernise network_interfaces

It's not 1996 anymore, and we use CIDR nowadays. Update the various
ifconfig_ examples to use CIDR notation instead of netmasks, and also
add an example of a basic ifconfig_ entry that most users will be
interested in.

Reviewed by: imp
Pull Request: https://github.com/freebsd/freebsd-src/pull/1217