new Portfile lldpd: LLDP Daemon and client

[lldpd](https://lldpd.github.io) is a daemon that implements LLDP and
LLDP-MED.

LLDP (Link Layer Discovery Protocol) is a link-local protocol—that is,
it's limited to the physical Ethernet connection between one device and
another—that devices use to exchange information about how they are
connected, and what their capabilities are, from a Layer-1 and Layer-2
perspective.

For example, say your Mac has a physical Ethernet connection that is
plugged into a switch.  If both sides support LLDP, the switch might
tell you:

* Its make, model, and OS.
* The name of the switch, and its management IP address.
* What type of Ethernet connections are supported (you may be on a port
  that can support 10-Gigabit Ethernet).
* What VLAN you are on, and for tagged (802.11q) VLANs, what VLANs are
  being provided.

It can be useful for sysadmins who are trying to figure out where an
Ethernet cable goes (assuming you can unplug it): Plug one end into your
Mac's Ethernet adapter, and within a minute you'll likely get the
information.

VoIP phones are prominent users of LLDP, identifying themselves as IP
phones.  Switches can react to this information by enabling VLAN tagging
on a port, telling the phone which VLAN is to be used for voice traffic.
These are examples of LLDP Media Endpoint Discovery (LLDP-MED).

lldpd also implements several related protocols—such as the
proprietary Cisco Discovery Protocol (CDP)—that have largely been
supplanted by LLDP.

There are three components:

* `lldpd` is a daemon that connects to all supportable (Ethernet)
  interfaces, sending out LLDP messages and listening for incoming
  messages.  It is launched via a LaunchDaemon.

* `lldpctl` can be used to see what information is known about connected
  devices.

* `lldpcli` can provide the same information as `lldpctl`, and can also
  be used to configure `lldpd`.

Closes: https://trac.macports.org/ticket/50586
Closes: https://trac.macports.org/ticket/55122

net/lldpd/Portfile

@@ -0,0 +1,44 @@
+# -*- coding: utf-8; mode: tcl; tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 4 -*- vim:fenc=utf-8:ft=tcl:et:sw=4:ts=4:sts=4
+
+PortSystem            1.0
+PortGroup             github 1.0
+
+github.setup          lldpd lldpd 1.0.19
+revision              0
+categories            net
+maintainers           {@akkornel stanford.edu:akkornel} openmaintainer
+license               ISC
+
+description           DESC
+long_description      ${description}, LONGDESC
+
+homepage              https://lldpd.github.io
+
+depends_build         path:bin/pkg-config:pkgconfig
+
+# The distribution does bring its own libevent, which we could use instead of
+# the port.
+depends_lib           port:libevent \
+                      port:libxml2 \
+                      port:readline
+
+github.tarball_from   releases
+
+checksums             rmd160  76faefbcc0587dd6b5563bdd8c16c450a1118c28 \
+                      sha256  4de17fe5137b4d44a7bd57f8dfc80cffe2c8bb3691b4ae3012b5a6ea20d79ee0 \
+                      size    1987102
+
+# libevent, libxml2, and readline are detected automatically by configure
+configure.args        --prefix=${prefix} \
+                      --with-privsep-chroot=/var/empty \
+                      --with-privsep-user=nobody \
+                      --with-privsep-group=nogroup \
+                      --with-launchddaemonsdir=no
+
+build.target           all-recursive
+
+installs_libs          no
+
+startupitem.create     yes
+startupitem.autostart  yes
+startupitem.executable ${prefix}/sbin/lldpd