net/enet: update to 1.3.18

https://github.com/lsalzman/enet/blob/master/ChangeLog

 * Packet sending performance improvements
 * MTU negotiation fixes
 * Checksum alignment fix
 * No more dynamic initialization of checksum table
 * ENET_SOCKOPT_TTL
 * Other miscellaneous small improvements

Reported by:	portscout