Add history section for devd(8)
Move sentence to a new line as advised by igor

PR:             212439
Approved by:    bcr (mentor)
Differential Revision:  https://reviews.freebsd.org/D8104

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

	Fix grammar bug.

Reported by:	ian
Sponsored by:	Spectra Logic Corporation

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.

Sponsored by:	Spectra Logic

sbin/devd/devd.cc
	Add a -q flag to devd that will suppress syslog logging at
	LOG_NOTICE or below.

Requested by:	ian@ and imp@
MFC after:	3 weeks
Sponsored by:	Spectra Logic Corporation

sbin/devd/devd.cc
        All output will now go to syslog(3) if devd is daemonized, or stderr
        if it's running in the foreground.

sbin/devd/devd.8
        Remove the "-D" flag.  Filtering messages by priority now
	happens in the usual syslog way.  For performance reasons, a few
	extra-verbose debugging statements are now conditional on the "-d" (do
	not daemonize) flag.

etc/syslog.conf
etc/newsyslog.conf
        Direct messages from devd(8) to /var/log/devd.log, but leave it
        disabled by default

Reviewed by:	eadler
Approved by:	gibbs (co-mentor)
MFC after:	never (removed a command-line option from devd)

Approved by:	joel (mentor)

their socket connection any time, and devd only notices that when it gets an
error trying to write an event to the client.  On a system with no device
change activity, clients could connect and disappear repeatedly without devd
noticing, leading to an ever-growing list of open socket descriptors in devd.

Now devd uses poll(2) looking for POLLHUP on all existing clients every time
a new client connection is established, and also periodically (once a minute)
to proactively find zombie clients and reap the socket descriptors.  It also
now has a connection limit, configurable with a new -l <num> command line arg.
When the maximum number of connections is reached it stops accepting new
connections until some current clients drop off.

Reviewed by:	imp
Approved by:	cognet (mentor)

Pointy hat to:	glebius