postfix / gateway: add bysender map
    
    This is a hack to work around SPF screwing us for @fedoraproject.org
    aliases. It only fixes email from @redhat.com, but due to bugzilla thats
    a lot of email.
    
    Without this:
    bugzilla@redhat.com -> user@fedoraproject.org (expands) ->
    user@gmail.com sent out directly to gmail and gets rejected because
    we aren't in the redhat.com SPF record.
    
    With this:
    
    bugzilla@redhat.com -> user@fedoraproject.org (expands) ->
    user@gmail.com but sent to mx2.redhat.com to deliver. Since
    mx2.redhat.com definitely is in the redhat.com SPF record the email is
    delivered fine and SPF checks pass.
    
    This won't help for other domains with -all SPF records, but at least it
    helps for all the redhat.com emails, of which there are a lot going to
    fedoraproject.org aliases. :)
    
    Signed-off-by: Kevin Fenzi <kevin@scrye.com>