Mark Terminate() and a few more functions as noreturn

[magv]

This helps the compiler identify which code paths are taken rarely, and move them out of the way from hot paths. I'd expect a marginal (below 1%) performance improvement, if there was a benchmark suite. The NORETURN macro currently expands into __attribute__((noreturn)), which GCC and clang understand, or nothing, if the configure script figured out that the compiler doesn't know the syntax. In principle, C11 and C++11 both support a noreturn attribute (with different syntax), but I wasn't sure what is the minimum language version requirement.

There's the issue of the Crash() function that crashes or not depending on the flags. I did not mark it as noreturn. I think that one deserves a separate consideration (i.e. why is it used in sort.c?).

[coveralls]

coverage: 58.027% (+0.1%) from 57.893%
when pulling 75b5da9 on magv:master
into eb24708 on form-dev:master.

[jodavies]

I don't measure any performance difference here indeed, but that doesn't mean we shouldn't commit "performance best practice".

I would say that Crash should not be called directly, and only via Terminate. There are only the two calls in sort.c to replace: these are both fatal errors.

[jodavies]

@magv can you fix the conflicts since the merge of #674 ?