boot: Move boot source code /usr/src/stand.

sys/boot becomes stand/boot and lib/libstand becomes stand/lib.

My main reason is to have boot out of the way when for example grepping
in sys/. It also

boot: Move boot source code /usr/src/stand.

sys/boot becomes stand/boot and lib/libstand becomes stand/lib.

My main reason is to have boot out of the way when for example grepping
in sys/. It also makes more sense since it's not really userland or
kernel code.

Other BSDs have moved their boot/standalone code around too, but maybe
with different directory layouts.

Approved-by: dillon, zrj

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