Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Update to latest git version of dtc to get new dtsv2 support,
including the include directive.

Fix minor build issue corrected by converting yypush_buffer_state and
yypop_buffer_state to yy_set_buffer_state and a hard-coded 100-deep
stack. It was easier to fix it here than to import that support into
our flex.

The new tools and test hardness remain unsupported at the moment.

Add missing files in f807af192828222dee7a5c9f94d999673bb4d8a1 import.