This command adds in the "fscrypt metadata" command. This command allows
advanced users to manipulate the metadata directly instead of just
creating a policy or protector as an option when encrypting a directory.

As some of these methods will require certain flags, error handling for
this case is also added. As the change passphrase method must indicate
when a old vs new password is necessary, additional KeyFuncs are added
which add this indicator.

Change-Id: Ibc92872088fae078df3c0eebd4f0cfcb7252d781

This commit adds in the status command, which has 2 functions, allowing
the user to query the state of the entire system or a specific
filesystem.

This commit also adds in the purge command to remove all policy keys
corresponding to a filesystem. This (along with getting the unlock
status for the status commands) uses additional keyctl functionality in
the crypto and actions packages.

Change-Id: Ic8e097b335c044c0b91973eff19753f363f4525d

This commit adds in the framework for adding commands and subcommands to
the fscrypt tool. This commit adds in the "setup", "encrypt", and
"unlock" commands. Additional information can be found by running:
    fscrypt <command> --help.

This commit defines how flags are parsed and errors are handled. It also
creates an extensible framework for prompting the user for information.

Change-Id: I159d7f44ee2b2bbc5e072f0802850e082d9a13ce

This commit adds in a stub fscrypt program. The binary just tells the
time and the tests do nothing, but the Makefile will build them! This
commit also adds documentation to the README that explains how to get,
build, run, test, format, lint, and install the code.

Also note that the executable is now in the cmd/fscrypt directory. The
library implementing the core functionality will be at the root. This
is essentially point 2 of https://medium.com/@benbjohnson/structuring-applications-in-go-3b04be4ff091

Change-Id: Ib7bd782e458bdf3db456beb978be4c75b4734561