Modify the Hoare judgment such that is also ensures thread safity.

Before, it just ensured that whenever one can split up the memory
into two parts before running the program, the framing part can
be split off when the program is finished. Now it also ensure that
it can be split off at any moment during the execution of the
program.

When we extend to non-deterministic expressions/sequence points we
certainly need this for the Hoare judgment for expressions, as C
allows evaluation of them to interleave. However, for consistency,
and further extensions, it is nice to have this property for
statements too.