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");
     }
 }
 

unix/uxnet.c

@@ -454,7 +454,7 @@ 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
     struct in_addr a;
 
@@ -723,7 +723,7 @@ static int try_connect(NetSocket *sock)
 	break;
 
       default:
-	assert(0 && "unknown address family");
+	unreachable("unknown address family");
 	exit(1); /* XXX: GCC doesn't understand assert() on some systems. */
     }
 

unix/uxpgnt.c

@@ -201,7 +201,7 @@ void pageant_print_env(int pid)
                socketname, pid);
         break;
       case SHELL_AUTO:
-        assert(0 && "Can't get here");
+        unreachable("SHELL_AUTO should have been eliminated by now");
         break;
     }
 }
@@ -708,7 +708,7 @@ void run_client(void)
             }
             break;
           default:
-            assert(0 && "Invalid client action found");
+            unreachable("Invalid client action found");
         }
     }