Importing mdata-client

[Joerg Sonnenberger <joerg%bec.de@localhost>]

Hello,
I'd like to import mdata-client into base for simplified creation of
Joyent-friendly images. The mdata protocol provides a simple
configuration protocol for a KVM/native zone to query optional
parameters. It is often use for providing e.g. a ssh authorized_keys
file, but can also be used for other parameters like querying the host
name etc. In the KVM case, it is implemented using a serial port.

While the protocol itself is currently implemented only by SmartOS, it
is well documented and could easily be used by other qemu-using
deployments, both natively and cross-target. The size of the source is
below 70KB, including man pages. I currently don't want to install the
programs unconditionally, but under a special build flag like MKJOYENT
or whatever name fits. This flag has a number of other side effects:
(1) Default to serial console for the ISO image.
(2) Adjust dhcpcd default configuration to use MAC as client identifier
and MAC-derived SLAAC.
(3) Provide /etc/rc.d/mdata to interprete the user_script option if
present.
(4) In case of netbsd-7: allow key-based root login.

Automatic resizing of root for rpi-like boot images would also be nice,
but that's a question for a latter point.

Joerg