udev works: /sys device tree, uevent netlink, datagram sockets with per-message credentials

Kernel devices (input, mem, tty, graphics, block) get /sys/devices/virtual
entries with dev/uevent/subsystem, /sys/class and /sys/dev links, input
capabilities, and writable uevent files that announce on the netlink socket.
eudev now runs, processes a full trigger and settles.

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

include/net/socket.h

@@ -344,6 +344,11 @@ struct socket {
   // AF_NETLINK only: which protocol, and the multicast groups bound.
   int   nlproto;
   uint  nlgroups;
+  uint  nlpid;                  // port id, assigned at bind
+  uint  nllast;                 // groups of the datagram read last
+  uint  nlfrom;                 // port id (netlink) or pid (Unix) of its sender
+  uint  pk_uid, pk_gid;         // Unix packet: credentials of its sender
+  int   pk_valid;               // a packet has been read
 
   // Connections waiting to be accepted, on a listener.
   struct socket *queue[NBACKLOG];