[MINOR] permit renaming of x-forwarded-for header

[MINOR] permit renaming of x-forwarded-for header

Because I needed it in my situation - here's a quick patch to
allow changing of the "x-forwarded-for" header by using a suboption to
"option forwardfor".

Suboption "header XYZ" will set the header from "x-forwarded-for" to "XYZ".

Default is still "x-forwarded-for" if the header value isn't defined.
Also the suboption 'except a.b.c.d/z' still works on the same line.

So it's now: option forwardfor [except a.b.c.d[/z]] [header XYZ]