net/wireguard: Introduce general wireguard_env parameter for environment
variables which affect wg-quick or the userspace implementation (eg.
wireguard-go). The syntax in rc.conf is:

wireguard_env="VAR=val VAR2=val2"

As a reference wg-quick uses these environment variables

WG_QUICK_USERSPACE_IMPLEMENTATION

and wireguard-go

LOG_LEVEL
WG_TUN_FD
WG_UAPI_FD
WG_PROCESS_FOREGROUND

Submitted by: J.R. Oldroyd <fbsd@opal.com>