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.

CR:		https://reviews.freebsd.org/rS266519
MFC after:	5 days
Sponsored by:	Spectra Logic