cmd/geth, cmd/utils: improve input handling

These changes make prompting behave consistently on all platforms:

* The input buffer is now global.
  Buffering was previously set up for each prompt, which can cause weird
  behaviour, e.g. when running "geth account update <input.txt" where
  input.txt contains three lines. In this case, the first password
  prompt would fill up the buffer with all lines and then use only the
  first one.

* Print the "unsupported terminal" warning only once.
  Now that stdin prompting has global state, we can use it to track
  the warning there.

* Work around small liner issues, particularly on Windows.
  Prompting didn't work under most of the third-party terminal emulators
  on Windows because liner assumes line editing is always available.

cmd/geth/js_test.go

@@ -94,7 +94,7 @@ func testREPL(t *testing.T, config func(*eth.Config)) (string, *testjethre, *nod
 		t.Fatal(err)
 	}
 	// Create a networkless protocol stack
-	stack, err := node.New(&node.Config{PrivateKey: testNodeKey, Name: "test", NoDiscovery: true})
+	stack, err := node.New(&node.Config{DataDir: tmp, PrivateKey: testNodeKey, Name: "test", NoDiscovery: true})
 	if err != nil {
 		t.Fatalf("failed to create node: %v", err)
 	}