BUG/MINOR: backend: properly handle redispatch 0

BUG/MINOR: backend: properly handle redispatch 0

According to the documentation, "option redispatch 0" is expected to
disable redispatch just like "no option redispatch", but due to the
fact that it keeps PR_O_REDISP set, it doesn't actually work. Let's
make sure value 0 is properly handled and drops PR_O_REDISP. This can
be backported to all versions since it seems it has been broken since
its introduction in 1.6 with commit 726ab7145c ("MEDIUM: backend: Allow
redispatch on retry intervals").

As a workaround, "no option redispatch" does work though.