mount: replace rpassword with rustix::termios

[lotheac]

because rpassword unconditionally open()s /dev/tty, it fails with ENXIO on the console without workarounds like busybox's cttyhack. in contrast, bcachefs unlock works fine on console, so change the passphrase prompt logic in mount to be closer to what it is in unlock.

[lotheac]

fixes the problem reported previously as https://github.com/koverstreet/bcachefs-tools/issues/292 ; the crucial part is that in the initrd, /dev/console is in fact a tty (and thus the prompt code path is taken), but it is not a controlling terminal and cannot be open()ed.

[tmuehlbacher]

rustix is already in the dependency tree, the nix crate is not.

[lotheac]

rustix is already in the dependency tree, the nix crate is not.

thanks. replaced with rustix::termios.

[koverstreet]

Merged