AF_UNIX SEQPACKET/DGRAM, writable /dev overlay, uevent netlink socket; eudev packages

udevd now starts, reads its rules and listens. Still to come: the sysfs
device tree and writable uevent files so a trigger has something to replay.

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

include/net/socket.h

@@ -24,10 +24,14 @@
 #define AF_NETLINK      16
 #define PF_NETLINK      AF_NETLINK
 #define NETLINK_ROUTE   0
+#define NETLINK_KOBJECT_UEVENT 15
+#define UEVENT_GROUP_KERNEL 1   // what the kernel announces
+#define UEVENT_GROUP_UDEV   2   // what udevd re-announces after its rules
 
 #define SOCK_STREAM     1
 #define SOCK_DGRAM      2
 #define SOCK_RAW        3
+#define SOCK_SEQPACKET  5
 
 // Flags ORed into socket()'s and accept4()'s `type`/`flags` argument. Linux's
 // values, because a musl program passes Linux's constants.
@@ -337,6 +341,10 @@ struct socket {
   char  data[SOCKBUF];
   uint  nread, nwrite;
 
+  // AF_NETLINK only: which protocol, and the multicast groups bound.
+  int   nlproto;
+  uint  nlgroups;
+
   // Connections waiting to be accepted, on a listener.
   struct socket *queue[NBACKLOG];
   int   nqueue;