MFC r343005: Handle SIGIO for listening sockets

r319722 separated struct socket and parts of the socket I/O path into
listening-socket-specific and dataflow-socket-specific pieces. Listening
socket connection notifications are now handled by solisten_wakeup() instead
of sowakeup(), but solisten_wakeup() does not currently post SIGIO to the
owning process.

PR: 234258