MFC devd-related changes

MFC devd-related changes

r270004
Convert devd's client socket to type SOCK_SEQPACKET.

  This change consists of two merges from projects/zfsd/head along with the
  addition of an ATF test case for the new functionality.

  sbin/devd/tests/Makefile
  sbin/devd/tests/client_test.c
        Add ATF test cases for reading events from both devd socket types.

  r266519:
  sbin/devd/devd.8
  sbin/devd/devd.cc
        Create a new socket, of type SOCK_SEQPACKET, for communicating with
        clients. SOCK_SEQPACKET sockets preserve record boundaries,
        simplying code in the client. The old SOCK_STREAM socket is retained
        for backwards-compatibility with existing clients.

  r269993:
  sbin/devd/devd.8
        Fix grammar bug.

r270019 (from bz)
Remove bogus ; at the end of the if condition in order to unbreak gcc builds
  after r270004.

  MFC after:    4 days
  X-MFX with:   r270004

git-svn-id: svn://svn.freebsd.org/base/stable/10@270242 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f