kevent: Restore old EV_EOF semantics

kevent: Restore old EV_EOF semantics

- EV_EOF should be set when the other side closed the connection, even
  if there are data pending in the read buffer (the old semantics).
- EV_NODATA is added to indicate there are no more data pending in the
  buffer and EOF is detected (EV_EOF is also set in this situation).

Kernel code now tests EV_NODATA instead of EV_EOF, since EV_NODATA
delivers the information which was delivered by the EV_EOF before
this commit.

DragonFly-Bug: http://bugs.dragonflybsd.org/issue1998