Copy the GitHub release notes into a NEWS.md file so that the release
notes are included in the actual git repo.  This way, they aren't hidden
away in GitHub, where they require an internet connection to access and
will be lost if GitHub ever goes away.  This also makes the release
notes be properly versioned; GitHub allows past release notes to be
edited, and there doesn't seem to be any record of what changed.

Finally, this allows packages to install the release notes into
/usr/share/doc/$pkgname/, as is the usual convention.

Now that it's been requested by users, bring back the "unlock_only"
option, which was originally proposed as part of
https://github.com/google/fscrypt/pull/281 but was dropped in the final
version of that pull request.

Resolves https://github.com/google/fscrypt/issues/357

Support for AES_256_HCTR2 filenames encryption was added in kernel
version 6.0.  The kernel doesn't yet support AES_256_HCTR2 for contents
encryption.

Bump up the required Go version to 1.16 so that we can assume that Go
modules are enabled by default.  Go 1.16 is the latest end-of-life
release, so this makes it so that we support the latest end-of-life
release (1.16), the current maintainance release (1.17), the current
release (1.18), and future releases.  This the same approach we took
when we last bumped up the required Go version.

Also update the ci.yml file to test with these versions.

Currently, fscrypt policies and protectors are world readable, as they
are created with mode 0644.  While this can be nice for use cases where
users share these files, those use cases seem to be quite rare, and it's
not a great default security-wise since it exposes password hashes to
all users.  While fscrypt uses a very strong password hash algorithm, it
would still be best to follow the lead of /etc/shadow and keep this
information non-world-readable.

Therefore, start creating these files with mode 0600.

Of course, if users do actually want to share these files, they have the
option of simply chmod'ing them to a less restrictive mode.  An option
could also be added to make fscrypt use the old mode 0644; however, the
need for that is currently unclear.

The metadata validation checks introduced by the previous commits are
good, but to reduce the attack surface it would be much better to avoid
reading and parsing files owned by other users in the first place.

There are some possible use cases for users sharing fscrypt metadata
files, but I think that for the vast majority of users it is unneeded
and just opens up attack surface.  Thus, make fscrypt (and pam_fscrypt)
not process policies or protectors owned by other users by default.
Specifically,

   * If fscrypt or pam_fscrypt is running as a non-root user, only
     policies and protectors owned by the user or by root can be used.

   * If fscrypt is running as root, any policy or protector can be used.
     (This is to match user expectations -- starting a sudo session
     should gain rights, not remove rights.)

   * If pam_fscrypt is running as root, only policies and protectors
     owned by root can be used.  Note that this only applies when the
     root user themselves has an fscrypt login protector, which is rare.

Add an option 'allow_cross_user_metadata' to /etc/fscrypt.conf which
allows restoring the old behavior for anyone who really needs it.

World-writable directories are not appropriate for some systems, so
offer a choice of single-user-writable and world-writable modes, with
single-user-writable being the default.  Add a new documentation section
to help users decide which one to use.

Fixes https://github.com/google/fscrypt/issues/318