pam: checking a user's login passphrase

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

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