Also add go version attrubute to go.mod

Show the encryption options when running 'fscrypt status' on a
directory.  E.g.:

	Policy:   490515286453d3f7
	Options:  padding:32 contents:Adiantum filenames:Adiantum
	Unlocked: Yes

* filesystem: ensure data is persisted before returning success

Sync the temporary file before renaming it, to ensure that after a
crash, the destination file isn't zero-length or otherwise incomplete.

Also sync the directory after the rename, to ensure the rename has been
persisted before returning success.

* filesystem: don't use fixed temporary file name

Using a fixed temporary file name in a world-writable sticky directory
is problematic since another user can create the file first.

Use ioutil.TempFile() to do it properly.  It uses O_EXCL under the hood
to ensure the file is newly created.

These were found by a combination of manual review and a custom script
that checks for common errors.

Also removed an outdated sentence from the comment for setupBefore().

Add the tags file to .gitignore, for developers using
https://github.com/jstemmer/gotags.

Fix 'make format'-related CI failure

This fixes a CI failure, caused by goimports changing how it formats the
imports.

Due to a goimports update, 'make format' is now changing metadata.pb.go.
But this fix can't be committed because this file is generated by
'make gen'.

Fix this by not formatting generated files.

Resolves https://github.com/google/fscrypt/issues/124

Resolves https://github.com/google/fscrypt/issues/117
Resolves https://github.com/google/fscrypt/issues/127