[wip] devel/libmsp430

Port for Texas Instruments MSP430 debugging interface library.
It can be used by devel/mspdebug to program an MSP MCU through
a supported programming tool. However devel/mspdebug does not
depend on libmsp430 to be functional.

I think this port is still not ok to commit; these issues must
be solved first:

(1) It uses several licenses, most free (MIT, BSD, Unlicense),
except for one, that I am not certain of. The source file
is distributed with an HTML manifest file (attached), that
contains the "TI Texas File", a limited license from Texas
Instruments (aka TI). But other sources (as the changelog
at homepage) say it is licensed under the "TI TSPA". I am
not sure which license is the correct one, or whether they
permit distribution of binaries not provided by TI itself.

(2) TI distributes the library under the name "MSP Debug Stack
Open Source Package" (or MSPDS). Other systems package it
as "msp430-debug-stack" (FreeBSD) or "mspds" (Arch Linux).
I chose "libmsp430" because that's it what the port builds
and installs (with its header files): `libmsp430.so`.

(3) The library finds a serial device matching an attached USB
device with known vendor and product ID. In order to find
both usb(4) device and ucom(4) port, I wrote a patch which
uses the `usb_device_info` structure and the new sysctl(2)
`hw.ucomnames` MIB (added in OpenBSD 7.4). It's the first
time I use both interfaces, so I ask to review it.

I cannot set myself as maintainer. I do not have access to an
MSP MCU or its programmer tools forever. I am working with an
experimenter board (MSP-EXP430FR5969) at day job. It is owned
by the company, so I do not have free access to it to test and
patch the port for much time.

Attached are the port tarball and manifest file with licenses.

Port homepage: http://www.ti.com/tool/MSPDS

Comments?

-- Lucas de Sena