net/netembryo: Add 2 new options, SCTP and IPv6

DragonFly does not support SCTP. This was detected fine in the configure
script, but check-plist failed due to an SCTP file that didn't get
installed. The configure script takes options for SCTP and IPv6, so turn
those into options. Use the brand-new OPTIONS_EXCLUDE_DragonFly feature
to allow SCTP to be on by default for FreeBSD and not even an option for
DragonFly.