ecryptfs mount of private should prompt for mount passphrase if not in keyring

[Dustin Kirkland ]

Extracted from Bug #268014.

The shell wrapper script ecryptfs-mount-private could be extended to accept a new parameter (or possibly default to it), to attempt to mount.ecryptfs_private with key(s) in the kernel keyring, and if not found, interactively prompt the user to enter the mount passphrase, add to the keyring, and retry.

The workflow looks like this:
 * user requests a mount of encrypted ~/Private
 * if it's already mounted, exit
 * try to mount with the key(s) currently in the user's keyring (see: keyctl show), exit on success
 * key is not in the keyring, so ask the user if they know the mount passphrase
 * prompt for mount passphrase and add to keyring (see: ecryptfs-add-passphrase)
 * retry the mount

:-Dustin