High level Rust API to OpenBSD's readpassphrase function.

From man 3 readpassphrase:

``` The readpassphrase() function displays a prompt to, and reads in a passphrase from, /dev/tty. If this file is inaccessible and the RPPREQUIRETTY flag is not set, readpassphrase() displays the prompt on the standard error output and reads from the standard input. In this case it is generally not possible to turn off echo.

Up to bufsiz - 1 characters (one is for the NUL) are read into the
provided buffer buf.  Any additional characters and the terminating
newline (or return) character are discarded.

The flags argument is the bitwise OR of zero or more of the following
values:

      RPP_ECHO_OFF            turn off echo (default behavior)
      RPP_ECHO_ON             leave echo on
      RPP_REQUIRE_TTY         fail if there is no tty
      RPP_FORCELOWER          force input to lower case
      RPP_FORCEUPPER          force input to upper case
      RPP_SEVENBIT            strip the high bit from input
      RPP_STDIN               read passphrase from stdin; ignore prompt

```