Close comment.
From Henning Petersen in PR 49403.

Build fix.

Switch libdevmapper library to use libdm as it;s backend lib and do not
communicate with device-mapper directly. Our lvm stack now looks like this
lvm<->libdevmapper<->libdm<->dm where only libdm kno

Switch libdevmapper library to use libdm as it;s backend lib and do not
communicate with device-mapper directly. Our lvm stack now looks like this
lvm<->libdevmapper<->libdm<->dm where only libdm knows how our dm protocol
looks like.

No objections on tech-userlevel@.

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Remove duplicated netbsd-dm.h file it is already installed in include/dev/dm.
Add own libddevmapper library header file libdm-netbsd.h.