BUG/MEDIUM: listener: do not try to rebind another process' socket

BUG/MEDIUM: listener: do not try to rebind another process' socket

When the "process" setting of a bind line limits the processes a
listening socket is enabled on, a "disable frontend" operation followed
by an "enable frontend" triggers a bug because all declared listeners
are attempted to be bound again regardless of their assigned processes.
This can at minima create new sockets not receiving traffic, and at worst
prevent from re-enabling a frontend if it's bound to a privileged port.

This bug was introduced by commit 1c4b814 ("MEDIUM: listener: support
rebinding during resume()") merged in 1.6-dev1, trying to perform the
bind() before checking the process list instead of after.

Just move the process check before the bind() operation to fix this.
This fix must be backported to 1.7 and 1.6.

Thanks to Pavlos for reporting this one.