Merge from projects/sendfile:

Provide pru_ready for AF_LOCAL sockets.  Local sockets sendsdata directly
to the receive buffer of the peer, thus pru_ready also works on the peer
socket.

Sponsored by:	Netflix
Sponsored by:	Nginx, Inc.


git-svn-id: svn+ssh://svn.freebsd.org/base/head@275332 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f
Merge from projects/sendfile:

- Provide pru_ready function for TCP.
- Don't call tcp_output() from tcp_usr_send() if no ready data was put
  into the socket buffer.
- In case of dropped connection don't try to m_freem() not ready data.

Sponsored by:	Nginx, Inc.
Sponsored by:	Netflix


git-svn-id: svn+ssh://svn.freebsd.org/base/head@275333 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f