Netlink datagrams can be measured, credentials name their sender

Epiphany's main thread spun at 100% on GLib's network monitor. GLib
sizes each netlink datagram with recvmsg(MSG_PEEK|MSG_TRUNC) and no
buffer; that answered 0, GLib read 0 bytes, the answer stayed queued and
poll() said readable again, forever. recvmsg on netlink now peeks
without consuming and with MSG_TRUNC returns what is waiting. A drained
netlink socket also no longer reports hangup -- it has no peer, and
sock_hup took that for a closed one.

SCM_CREDENTIALS under SO_PASSCRED named the process that created the far
end, not the one that wrote to it. For a socketpair those differ: WebKit
makes the pair, hands one end to a child, and learns the child's pid
from the credentials on its first byte. Each end now records its last
writer.

LDLS: napi_consume_skb frees the skb. r8169 frees every sent frame with
it, and as a stub it leaked one per frame until the DMA heap ran out.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01S1giNvrHTrPMPw7V6x1kQ4

include/net/socket.h

@@ -362,6 +362,13 @@ struct socket {
   // the listener -- which is what Linux records too.
   int   cred_pid;
   uint  cred_uid, cred_gid;
+
+  // Who last wrote at this end, for SCM_CREDENTIALS on the other. Not the
+  // same thing as cred_*: one end of a socketpair is made by the parent and
+  // written by the child it is handed to, and WebKit learns its children's
+  // pids from exactly that. 0 until the first write.
+  int   tx_pid;
+  uint  tx_uid, tx_gid;
 };
 
 #endif