MINOR: proxy: make pr_mode enum bitfield compatible

MINOR: proxy: make pr_mode enum bitfield compatible

Current pr_mode enum is a regular enum because a proxy only supports one
mode at a time. However it can be handy for a function to be given a
list of compatible modes for a proxy, and we can't do that using a
bitfield because pr_mode is not bitfield compatible (values share
the same bits).

In this patch we manually define pr_mode values so that they are all
using separate bits and allows a function to take a bitfield of
compatible modes as parameter.