Chaning the --user flag to (optionally) check for a proper keyring setup
allows us to fail early in cases where we need a working keyring.

The --user flag can now be used to have the targe user (the one whose
keyring and password will be used in fscrypt) be different than the
calling user. Very usefull for things like

	sudo fscrypt purge /media/joerichey/usb --user=joerichey

which will now have privileges to drop caches, but will properly clear
the keys from the user's keyring.

Almost all actions only need to to check that the fscrypt metadata
exists (this is handled by the Mount methods). Only "fscrypt encrypt"
need to be sure the filesystem also supports encryption, so this check
is added.

Protectors are only reverted if they were created, and Policies are only
depovisioned on failure.

This commit changes all the internal import paths from `fscrypt/foo` to
`github.com/google/fscrypt/foo` so that it can be built once we release
externaly. The documentation in README.md is updated accordingly.

Also, the README has a note noting that we do not make any guarantees
about project stability before 1.0 (when it ships with Ubuntu).

Change-Id: I6ba86e442c74057c8a06ba32a42e17f94833e280

This commit adds in the "fscrypt metadata add-protector-to-policy" and
the "fscrypt metadata remove-protector-from-policy" subcommands. These
commands allow for the creating of policies protected by multiple
protectors.

Change-Id: Id7e6c057448d15757c838a82d487a1b9806f585d

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