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Also removed an outdated sentence from the comment for setupBefore().

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 changes the error handing for the crypto, filesystem,
metadata, pam, and util packages to use the error handling library
github.com/pkg/errors. This means elimination of the FSError type, an
increased use of wrapping errors (as opposed to logging), switching
on the Cause() of an error (as opposed to its value), and improving our
integration tests involving TEST_FILESYSTEM_ROOT.

This commit also fixes a few bugs with the keyring code to ensure that
our {Find|Remove|Insert}PolicyKey functions are always operating on the
same keyring. The check for filesystem support has been moved from the
filesystem package to the metadata package. Finally, the API for the
filesystem package has been slightly modified:
	* filesystem.AllFilesystems() now returns all the filesystems in
	  sorted order
	* certain path methods are now public

O_SYNC is also removed for writing the metadata. We don't get that much
from syncing the metadata, as the actual file data could also be
corrupted by and IO error. The sync operation is also occasionally very
slow (~3 seconds) and can be unfriendly to battery life.

Change-Id: I392c2655141714b16dfdbc84ac09780072be2cf0

This commit adds in the fscrypt/pam package. This package will hold all
functionality related to Linux Pluggable Authentication Modules (PAM).
Right now this package uses cgo to mock a PAM conversation, allowing the
function to check if a provided passphrase actually belongs to a user.

Due to the nature of cgo callbacks, global state of the key to check is
necessary for this function. This commit also addresses some issues
about building the cgo components. Now, only the minimal linking flags
are included in the go files. Additional linker flags may now be
necessary to build a static binary of fscrypt. This is addressed in the
Makefile and README.

Finally, this commit fixes a bug where the tests would not run correctly
due to shared global state on the testing filesystem. Fixed, by having
all the tests run sequentially.

Change-Id: Ia43636801da984b505d2f43dd14127b7cfbf2c48