Convert a few more universal asserts to unreachable().

When I introduced the unreachable() macro in commit 0112936ef, I
searched the source code for assert(0) and assert(false), together
with their variant form assert(0 && "explanatory text"). But I didn't
search for assert(!"explanatory text"), which is the form I used to
use before finding that assert(0 && "text") seemed to be preferred in
other code bases.

So, here's a belated replacement of all the assert(!"stuff") macros
with further instances of unreachable().

sshshare.c

@@ -1292,8 +1292,7 @@ void share_got_pkt_from_server(ssh_sharing_connstate *cs, int type,
         break;
 
       default:
-        assert(!"This packet type should never have come from ssh.c");
-        break;
+        unreachable("This packet type should never have come from ssh.c");
     }
 }