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.

windows/window.c

@@ -1145,7 +1145,7 @@ static void update_mouse_pointer(void)
 	force_visible = true;
 	break;
       default:
-	assert(0);
+	unreachable("Bad busy_status");
     }
     {
 	HCURSOR cursor = LoadCursor(NULL, curstype);

windows/winnet.c

@@ -784,7 +784,7 @@ void sk_addrcopy(SockAddr *addr, char *buf)
 	    memcpy(buf, &((struct sockaddr_in6 *)step.ai->ai_addr)->sin6_addr,
 		   sizeof(struct in6_addr));
 	else
-	    assert(false);
+	    unreachable("bad address family in sk_addrcopy");
     } else
 #endif
     if (family == AF_INET) {

windows/winpgen.c

@@ -61,7 +61,9 @@ void nonfatal(const char *fmt, ...)
 }
 
 /* Stubs needed to link against misc.c */
-void queue_idempotent_callback(IdempotentCallback *ic) { assert(0); }
+void queue_idempotent_callback(IdempotentCallback *ic) {
+    unreachable("No callbacks in this application");
+}
 
 /* ----------------------------------------------------------------------
  * Progress report code. This is really horrible :-)

windows/winpgnt.c

@@ -86,7 +86,9 @@ void modalfatalbox(const char *fmt, ...)
 }
 
 /* Stubs needed to link against misc.c */
-void queue_idempotent_callback(IdempotentCallback *ic) { assert(0); }
+void queue_idempotent_callback(IdempotentCallback *ic) {
+    unreachable("No callbacks in this application");
+}
 
 static bool has_security;
 

windows/winpgntc.c

@@ -27,7 +27,7 @@ bool agent_exists(void)
 
 void agent_cancel_query(agent_pending_query *q)
 {
-    assert(0 && "Windows agent queries are never asynchronous!");
+    unreachable("Windows agent queries are never asynchronous!");
 }
 
 agent_pending_query *agent_query(