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
Reported by: Kenneth Adelman
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D18664