Add notpossible keyword

This is the same as `unreachable` except it always traps violations,
even if we're not running in MODE=dbg. This is useful for impossible
conditions relating to system calls. It avoids terrifying bugs where
control falls through to an unrelated function.

tool/emacs/cosmo-c-keywords.el

@@ -61,6 +61,7 @@
            "reallocesque"
            "nullterminated"
            "unreachable"
+           "notpossible"
            "hidden"
            "privileged"
            "hasatleast"