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/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");
         }
     }