s3: Make winbind recover from a signing error

s3: Make winbind recover from a signing error

When winbind sees a signing error on the smb connection to a DC (for whatever
reason, our bug, network glitch, etc) it should recover properly. The "old"
code in clientgen.c just closed the socket in this case. This is the right
thing to do, this connection is spoiled anyway. The new, async code did not do
this so far, which led to the code in winbindd_cm.c not detect that we need to
reconnect.