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

ports/alpine/world

@@ -65,3 +65,17 @@ udev-init-scripts
 
 # Privilege for ordinary users: setuid doas (opendoas), configured in /etc/doas.d.
 doas
+
+# Input and seats for Wayland: libinput turns the evdev nodes udev tagged into
+# pointer, keyboard and touch events; libseat hands a compositor its devices
+# without making it root, through seatd's socket (or directly when it is root).
+# xf86-input-libinput is the same library for Xorg.
+libinput
+libinput-libs
+libinput-udev
+libseat
+seatd
+seatd-launch
+xf86-input-libinput
+# lsusb that reads /sys/bus/usb/devices, which busybox's does not.
+usbutils