atm: remove SVC socket support and the signaling daemon interface

atm: remove SVC socket support and the signaling daemon interface

ATM switched virtual circuits (SVCs) are set up and torn down by a
user-space signaling daemon (atmsigd) which the kernel talks to over
a dedicated "sigd" socket: the kernel marshals Q.2931-style requests
(as_connect, as_listen, as_accept, as_close, ...) to the daemon and
applies the results to PF_ATMSVC sockets. This is the machinery behind
classical SVC use and was the foundation for LANE / MPOA, all of which
have been removed.

DSL deployments do not use any of this. PPPoATM and BR2684 run over
permanent virtual circuits (PF_ATMPVC) with a statically configured
VPI/VCI; no atmsigd, no Q.2931. Neither remaining ATM driver
(solos-pci, the USB DSL modems) is reachable through the SVC path.

Remove the SVC socket family and the signaling interface:

 - delete net/atm/svc.c, net/atm/signaling.c and signaling.h
 - drop atmsvc_init()/atmsvc_exit() and the PF_ATMSVC registration and
   module alias
 - drop the ATMSIGD_CTRL ioctl (sigd_attach) and the /proc/net/atm/svc
   file
 - fold the SVC branch out of atm_change_qos(); all sockets are PVCs now

The obsolete ATM_SETSC ioctl stub is left in place (it already just
warns and returns 0), as is the struct atm_vcc SVC bookkeeping shared
with the queueing layer.

Link: https://patch.msgid.link/20260615194416.752559-6-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>