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

This fixes a test failure in the case where test.conf gets left over.

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

Running `crypto.PassphraseHash` in a loop allocates a lot of memory.
Golang is not always prudent about collecting the garbage from previous
runs, resulting in a OOM error on memory-pressured systems.

With a `maxMemoryBytes` of 128 MiB, this change reduces the maximum
resident memory for `fscrypt setup` to 141 MiB (was perviously 405 MiB)

Signed-off-by: Joe Richey <joerichey@google.com>

On systems with high memory pressure, using half of the entire RAM for
hashing can result in fscrypt getting OOM killed.

Signed-off-by: Joe Richey <joerichey@google.com>

Introduce filesystem.ErrEncryptionNotEnabled and
filesystem.ErrEncryptionNotSupported which include the Mount as context,
and translate the corresponding metadata/ errors into them.  Then make
these errors show much better suggestions.

Also replace lots of other filesystem/ errors with either custom types
or with unnamed one-off errors that include more context.  Fix backwards
wrapping in lots of cases.

Finally, don't include the mountpoint in places where it's not useful,
like OS-level errors that already include the path.

ErrMissingPolicyMetadata:
	Include the mount, directory path, and metadata path.  Also move
	the explanation into actions/ since it doesn't refer to any CLI
	command.

ErrPolicyMetadataMismatch:
	Include a lot more information.  Also start checking for
	consistency of the policy key descriptors, not just the
	encryption options.  Add a test for this.

ErrDifferentFilesystem:
	Include the mountpoints.

ErrOnlyProtector:
	Clarify the message and include the protector descriptor.

ErrAlreadyProtected:
ErrNotProtected:
	Include the policy and protector descriptors.

ErrAccessDeniedPossiblyV2:
	Make it slightly clearer what failed.  Also move the explanation
	into actions/ since it doesn't refer to any CLI command.

ErrProtectorName:
	Rename to ErrLoginProtectorName for clarity, and include the
	name and user.

ErrMissingProtectorName:
	Include the correct protector source.

ErrDuplicateName:
	Rename to ErrProtectorNameExists for clarity, and remove a level
	of wrapping by including the name directly.

ErrDuplicateUID:
	Rename to ErrLoginProtectorExists for clarity, and remove a
	level of wrapping by including the user directly.

ErrBadConfig:
	Fix backwards wrapping, include the bad config, and make it
	clear that this is an internal error.

ErrBadConfigFile:
	Fix backwards wrapping, include the config file location, and
	adjust the suggestion slightly.

ErrConfigFileExists:
	Include the config file location.

ErrNoConfigFile:
	Include the config file location, and adjust the suggestion
	slightly.

'fscrypt lock' on a v1-encrypted directory doesn't warn about in-use
files, as the kernel doesn't provide a way to easily detect it.

Instead, implement a heuristic where we check whether a subdirectory can
be created.  If yes, then the directory must not be fully locked.

Make both 'fscrypt lock' and 'fscrypt status' use this heuristic.

Resolves https://github.com/google/fscrypt/issues/215