ask-password: Add --echo to enable echoing the user input

ask-password: Add --echo to enable echoing the user input

Programs such as OpenVPN may use ask-password for not only retrieving
passwords, but also usernames.  Masking usernames with * seems just silly.

 v2 - Don't mess with termios flags, instead print the input
      instead of an asterix.  Resolves issues with backspace
      and TAB input.

 v3 - Renamed 'do_echo' variables and argument to 'echo'.  Also
      modified the ask_password_{tty,agent,auto} API instead of
      additional wrapper functions.

[zj: undo changes to ask_password_auto, since no callers were using
     the new argument.]