MFC r241161:

MFC r241161:

  Fix faulty error code handling in read(2) on TTYs.

  When performing a non-blocking read(2), on a TTY while no data is
  available, we should return EAGAIN. But if there's a modem disconnect,
  we should return 0. Right now we only return 0 when doing a blocking
  read, which is wrong.

MFC r242078:

  Correct SIGTTIN handling.

  In the old TTY layer, SIGTTIN was correctly handled like this:

        while (data should be read) {
                send SIGTTIN if not foreground process group
                read data
        }

  In the new TTY layer, however, this behaviour was changed, based on a
  false interpretation of the standard:

        send SIGTTIN if not foreground process group
        while (data should be read) {
                read data
        }

  Correct this by pushing tty_wait_background() into the ttydisc_read_*()
  functions.

  Reported by:  koitsu
  PR:           kern/173010

git-svn-id: svn://svn.freebsd.org/base/stable/9@242892 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f