net/enet: update to 1.3.16

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

• fix bug in unreliable fragment queuing
• use single output queue for reliable and unreliable packets for saner ordering
• revert experimental throttle changes that were less stable than prior algorithm

Reported by: portscout
MFH: 2020Q3 (runtime fixes)