std-aux: mark failures of nm_assert() as unreachable code

- with nm_assert(), if the argument is a compile time constant
  always check it (regardless of NDEBUG, G_DISABLE_ASSERT)
  and mark the failure as _nm_unreachable_code(). We do this,
  even if we usually would not evaluate run time checks with
  NDEBUG/G_DISABLE_ASSERT.

- with nm_assert_se(), if assertions are disabled with NDEBUG
  and G_DISABLE_ASSERT, still mark the path as _nm_unreachable_code().
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