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/chaincmd.go

@@ -116,7 +116,7 @@ func exportChain(ctx *cli.Context) {
 }
 
 func removeDB(ctx *cli.Context) {
-	confirm, err := utils.PromptConfirm("Remove local database?")
+	confirm, err := utils.Stdin.ConfirmPrompt("Remove local database?")
 	if err != nil {
 		utils.Fatalf("%v", err)
 	}