Expose connection info to up/down scripts via additional env variables

[wknapik]

# $trusted_ip/$untrusted_ip are not necessarily among the IPs a configured
# remote domain will resolve to. Some providers return a random subset out of
# hundreds of IPs on each query, and revdns entries are not guaranteed to
# exist, or match any remote as defined in the config.
current_remote := "1" | "2" | ...
or
current_remote := domain_name_or_ip_matching_configured_remote

# Consitent with $trusted_ip/$untrusted_ip, $trusted_port/$untrusted_port.
# Would allow setting precise firewall rules from an --up/--down script.
trusted_proto := "udp" | "tcp-client" | "tcp-server"
untrusted_proto := "udp" | "tcp-client" | "tcp-server"