Initial import of libdm and dmctl to tree. libdm library can be used
to access, manage and manipulate device-mapper driver. Which opens us bunch
of new possibilities like

dm-multipath device target
dm-crypt    device target
dm-ccd     compatibility layer

With this import I'm bringing in dmctl tool for working with dm driver ,too.
I plan to replace gpl2 licensed dmsetup command with our dmctl tool in near
feature. It can also by placed to /rescue where we was not able to put
dmsetup because of licensing problems.

With libdm in tree we can now write RUMP atf test suite for dm driver to
ensure LVM subsystem stability as time goes.

Reviewed by: blymn@ and rmind@
Oked: by no objections on tech-userlevel@

(haad)