[MEDIUM] add the "except" keyword to the "forwardfor" option

[MEDIUM] add the "except" keyword to the "forwardfor" option

Patch from Bryan Germann for 1.2.17.
In some circumstances, it is useful not to add the X-Forwarded-For
header, for instance when the client is another reverse-proxy or
stunnel running on the same machine and which already adds it. This
patch adds the "except" keyword to the "forwardfor" option, allowing
to specify an address or network which will not be added to this
header.