Import wireguard fixes from pfSense 2.5

Merge the following fixes from https://github.com/pfsense/FreeBSD-src
1940e7d3 Save address of ingress packets to allow wg to work on HA
8f5531f1 Fix connection to IPv6 endpoint
825ed9ee Fix tcpdump for wg IPv6 rx tunnel traffic
2ec232d3 Fix issue with replying to INITIATION messages in server mode
ec77593a Return immediately in wg_init if in DETACH'd state
0f0dde6f Remove unnecessary wg debug printf on transmit
2766dc94 Detect and fix case in wg_init() where sockets weren't cleaned up
b62cc7ac Close the UDP tunnel sockets when the interface has been stopped

Reviewed by: kevans
Obtained from: pfSense 2.5
MFC after: 3 days
Relnotes: yes
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D28962