Make remove-protector-from-policy work even if the protector cannot be
loaded (for example, due to having been deleted already).

Fixes https://github.com/google/fscrypt/issues/258
Fixes https://github.com/google/fscrypt/issues/272

"Linux filesystem encryption" sounds too vague.  Write "Linux native
filesystem encryption" instead.

Fixes #123

As per the feedback at https://github.com/google/fscrypt/issues/115
where users didn't understand that the recovery passphrase is important,
restore the original behavior where recovery passphrase generation
happens automatically without a prompt.  This applies to the case where
'fscrypt encrypt' is using a login protector on a non-root filesystem.

However, leave the --no-recovery option so that the recovery passphrase
can still be disabled if the user really wants to.  Also, clarify the
information provided about the recovery passphrase.

Update https://github.com/google/fscrypt/issues/115

In Linux 5.15, the no-key name format is changing again; see
https://git.kernel.org/linus/ba47b515f5940603.  isPossibleNoKeyName()
sometimes doesn't recognize the new no-key names.  Update it accordingly
to recognize all possible no-key names.

Note: isPossibleNoKeyName() is only used as a heuristic to check whether
a v1-encrypted directory is incompletely locked or not.  Therefore, it's
not too important whether it works.  However, this change is needed for
cli-tests/t_v1_policy to pass.

Fix the GRUB detection logic to take into account that
MOUNTPOINT/boot/grub might not be on the same filesystem as MOUNTPOINT,
due to MOUNTPOINT/boot being another mountpoint.  The warning is only
appropriate when GRUB is installed on the same filesystem that
encryption is going to be enabled on.

Fix word mismatch in usage and description of metadata create policy
command.

The golang.org/x/crypto/ssh/terminal package is deprecated and merely a
wrapper around golang.org/x/term. Thus, use the latter directly.

Update #272