Fix regression introduced by SVN r270954. Correct way to reset getopts
is to set OPTIND to 1, not unset it (which causes an error).

Thanks to:	jilles
Define _DTRACE_VERSION in sdt.h rather than setting it manually. This is
similar to what illumos does, and makes it easier to enable USDT probes
in third-party software that doesn't make use of the system makefiles.
Invoke make_dtb with MACHINE defined for enhanced cross building
friendliness. This should restore old-fashioned kernel building in a
cross environment, though this has only had limited testing.

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