x/term: ReadPassword not supported on plan9

[ncw]

x/crypto/ssh/terminal doesn't provide terminal.ReadPassword on plan9.

Related #13085

[minux]

Does Plan 9 even support terminal inputs without echo?

[9nut]

yes; echo rawon >/dev/consctl does it; from cons(3)

      "If the string rawon has been written to the consctl file and
      the file is still open, cons is in raw mode: characters are
      not echoed as they are typed, backspace, `^U' and `^D' are
      not treated specially, and characters are available to read
      as soon as they are typed.  Ordinary mode is reentered when
      rawoff is written to consctl or this file is closed."

[tklauser]

The golang.org/x/crypto/ssh/terminal package is deprecated and moved to golang.org/x/term, so support for ReadPassword on plan9 should be implemented in that package.