libinput, seatd and libseat; sockets from accept() can pass descriptors

Alpine's libinput, libseat, seatd, xf86-input-libinput and usbutils join the
image, and seatd runs from schinit. libinput lists the keyboard and pointer as
an ordinary user, Xorg now drives both through libinput, and a program linked
to libseat running as schos opens seat0 through seatd, is handed
/dev/input/event0 and reads its name through the descriptor.

Two kernel bugs stood in the way, and both would have stopped a Wayland
compositor too. The socket connect() queues for a listener was allocated with
its domain at zero, so sendmsg refused SCM_RIGHTS on the accepted end: a server
could receive descriptors and never send one. And sendmsg treated a control
buffer longer than its messages as an error, where Linux takes a following
header too short to be one as the end; seatd passes a zeroed CMSG_SPACE array
with one descriptor at the front.

Found with /proc/strace and a temporary kinfo in sendmsg's EINVAL sites.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EVRhxi9zpdW2uDUN4FVcao

kernel/net/unix.c

@@ -1112,6 +1112,12 @@ sockconnect(struct socket *s, uint dev, uint64 ino)
     return -ENFILE;
   }
 
+  // The accepted end is an AF_UNIX socket like the one that connected. It was
+  // left at domain 0, and nothing minded until a server passed a descriptor
+  // back through it: sendmsg refuses SCM_RIGHTS on a socket that is not
+  // AF_UNIX, so seatd could never hand over a device, and a Wayland
+  // compositor could never have sent its clients a buffer.
+  peer->domain = AF_UNIX;
   peer->peer  = s;
   peer->state = SS_CONNECTED;
   peer->cred_pid = srv->cred_pid;