Add support for specifying unsupported / broken options that override
any defaults or user specified actions on the command line. This would
be useful for specifying features that are always broken or that
cannot make sense on a specific architecture, like ACPI on pc98 or
EISA on !i386 (!x86 usage of EISA is broken and there's no supported
hardware that could have it in any event). Any items in
BROKEN_OPTIONS are forced to "no" regardless of other settings.
Clients are expected change BROKEN_OPTIONS with +=. It will not
be unset, so other parts of the build system can have visibility
into the options that are broken on this platform, though this
should be very rare.

Differential Revision: https://reviews.freebsd.org/D2009