Mark `help' and `version' functions as not returning.

Mark `help' and `version' functions as not returning.

They don't; and later versions of GCC complain about potential `switch'
fall-through mistakes without this.

Fixing this is a simple matter of adding `attribute((noreturn))' in the
right places.  Note that `server/gstdecode.c' doesn't share the common
attribute-macro machinery.