This patch adds support for exposing the environment variables
`TOR_PT_OUTBOUND_BIND_ADDRESS_V4` and `TOR_PT_OUTBOUND_BIND_ADDRESS_V6` to
Pluggable Transport proccesses. These two values will contain the IPv4
and IPv6 address that the user have specified in torrc that they wish
the PT to use for all outgoing IP packets.

It is important to note here that it is up to the indvidual Pluggable
Transport if they are willing to honor these values or ignore them
completely.

One can test this feature using the following dummy PT written in POSIX
shell script:

    #!/bin/sh

    echo "LOG SEVERITY=warning MESSAGE=\"Value for IPv4: ${TOR_PT_OUTBOUND_BIND_ADDRESS_V4}\""
    echo "LOG SEVERITY=warning MESSAGE=\"Value for IPv6: ${TOR_PT_OUTBOUND_BIND_ADDRESS_V6}\""

    while true ; do
        sleep 1
    done

with the following entries in your torrc:

    OutboundBindAddressPT 203.0.113.4
    OutboundBindAddress 203.0.113.5
    OutboundBindAddressPT 2001:db8::4
    OutboundBindAddress 2001:db8::5

See: https://bugs.torproject.org/5304