BUG/MINOR: sock: mark abns sockets as non-suspendable and always unbind them

BUG/MINOR: sock: mark abns sockets as non-suspendable and always unbind them

In 2.3, we started to get a cleaner socket unbinding mechanism with
commit f58b8db47 ("MEDIUM: receivers: add an rx_unbind() method in
the protocols"). This mechanism rightfully refrains from unbinding
when sockets are expected to be transferrable to another worker via
"expose-fd listeners", but this is not compatible with ABNS sockets,
which do not support reuseport, unbinding nor being renamed: in short
they will always prevent a new process from binding.

It turns out that this is not much visible because by pure accident,
GTUNE_SOCKET_TRANSFER is only set in the code dealing with master mode
and deamons, so it's never set in foreground mode nor in tests even if
present on the stats socket. However with master mode, it is now always
set even when not present on the stats socket, and will always conflict.

The only reasonable approach seems to consist in marking these abns
sockets as non-suspendable so that the generic sock_unbind() code can
decide to just unbind them regardless of GTUNE_SOCKET_TRANSFER.

This should carefully be backported as far as 2.4.