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.

github.com/golang/protobuf/proto has been deprecated in favor of
google.golang.org/protobuf/proto, so migrate to the non-deprecated one.

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.

When building pam_fscrypt.so, specify -buildmode=c-shared after
$(GO_FLAGS) so that it overrides any user-specified buildmode.

This is needed to allow -buildmode=pie to be specified in GO_FLAGS if
the packager wants to build fscrypt as a position-independent executable
(e.g. following https://wiki.archlinux.org/title/Go_package_guidelines).
Previously, trying to do this caused pam_fscrypt.so to be incorrectly
built as an executable rather than as a shared library.

This allows non Ubuntu distributions to opt out from the installation
of Ubuntu-specific PAM files.