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.
