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

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

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