Replace assert(false) with an unreachable() macro.

Taking a leaf out of the LLVM code base: this macro still includes an
assert(false) so that the message will show up in a typical build, but
it follows it up with a call to a function explicitly marked as no-
return.

So this ought to do a better job of convincing compilers that once a
code path hits this function it _really doesn't_ have to still faff
about with making up a bogus return value or filling in a variable
that 'might be used uninitialised' in the following code that won't be
reached anyway.

I've gone through the existing code looking for the assert(false) /
assert(0) idiom and replaced all the ones I found with the new macro,
which also meant I could remove a few pointless return statements and
variable initialisations that I'd already had to put in to placate
compiler front ends.

unix/gtkwin.c

@@ -679,7 +679,7 @@ static void update_mouseptr(GtkFrontend *inst)
                               inst->waitcursor);
 	break;
       default:
-	assert(0);
+	unreachable("Bad busy_status");
     }
 }