fcntl(2) lock on /dev/null fails with EINVAL

[Steffen Nurpmeso]

..and so here is a tiny manual update.
Just noting that locking succeeds on at least (my outdated) Snow
Leopard and i like that since /dev/null is the only sane, always
available option to foist something NULLy into whatever.
I.e., i use it to be able to have an available NULL mailbox for
the test (suite is a wording too strong, still).  Yes, it is
because the BSD Mail codebase i'm sitting upon still doesn't have
it own VOID object that can be addressed directly.

--steffen
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