This changes the vendored sources of github.com/golang/protobuf,
golang.org/x/crypto/ssh, and golang.org/x/sys/unix to be the current
master versions.

crypto/ssh/terminal gets us information about the terminal, such as
its dimensions and state. It also allows us to manipulate the terminal.
For example, we can put the terminal in raw mode when a passphrase needs
to be entered.

This commit also add the github.com/urfave/cli package which we will use
to write the command line tool component of fscrypt. This tool allows
for us to easily use commands and subcommands.

Note that this is actually the upstream repository with two PRs applied:

https://github.com/urfave/cli/pull/629
https://github.com/urfave/cli/pull/630

They fix bugs in the handling of custom usage errors.

Change-Id: I2d3ba967b1bce8f73440e3a06df4eaba7ab96c19

This commit adds in the golang.org/x/crypto/hkdf package which contains
the HMAC-based Extract-and-Expand Key Derivation Function (HKDF) as
defined in RFC 5869.

This package will be used later in the crypto package to stretch a
single cryptographic key into multiple keys.

Change-Id: I387230344bdeadc7d38c46994a228eecda2e12bd