if_ovpn: ensure we never re-use sequence numbers

if_ovpn already notified userpsace when there was a risk of sequence
number re-use, but it trusted userspace to actually rotate the key.

Convert the internal sequence number counter to 64 bits so we can detect
overflows and then refuse to send packets.

Event: BSDCan 2023
Reviewed by: Leon Dang <ldang@netgate.com>
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D40187