Add __attribute__((__noreturn__)) to various function prototypes in usr.bin/.

This general cleanup is useful in various ways. It helps the compiler
optimize better and it also helps later GCCs detec

Add __attribute__((__noreturn__)) to various function prototypes in usr.bin/.

This general cleanup is useful in various ways. It helps the compiler
optimize better and it also helps later GCCs detect better when to give
-Wimplicit-fallthrough warnings and when not.

Found-with: -Wsuggest-attribute=noreturn

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dfregress,tbridge - Move into usr.bin and sys/dev

* cleanup of the testcases, remove duplicates, consolidate in
test/testcases.