Fixup mi-nonstop.exp

[Vladimir Prus]

Pedro has noticed that the version of mi-nonstop.exp I've checked
is actually no-op, since the logic to detect if non-stop is available
was faulty. Dan has noticed that the corresponding C file produces
warnings (on 64-bit box, I presume). This patch fixes those; checked
in.

The test is still somewhat flaky, in particular in some runs gdb reports
sigtrap on breakpoint hit; but that's for another day.

- Volodya
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