According to:
https://www.gnu.org/software/coreutils/manual/html_node/arch-invocation.html
the arch command isn't portable, so we switch to uname.

Protoc: https://github.com/google/protobuf/blob/a711e3d5b4ee1dd7f9d21197dca8432a5819a64e/protoc-artifacts/build-protoc.sh#L82-L83 
Go: https://github.com/golang/sys/blob/37707fdb30a5b38865cfb95e5aab41707daec7fd/unix/linux/mkall.go#L43-L79

Using "arch" instead of "go env GOARCH" is more standard and doesn't
generate a warning on "sudo make install".

This change is a complete rewrite of fscrypt's Makefile.

The new build rules can be roughly divided into secions:
  Build   - bin/fscrypt and bin/pam_fscrypt
  Linting - gen (for .proto files), format, lint
  Test    - test, test-{setup|teardown}, coverage.out
  Install - install, uninstall, install-{bin|pam}
  Tools   - tools and other bin/* needed for the other rules

As before, "make" builds the binary and pam_module, while "make all"
builds and tests everything (except for integration tests), and "make
clean" removes any generated files.

Also note that this new build system:
  - Doesn't require input_fail.py
  - Properly falis on linter errors
  - Builds everything into the bin/ directory (customizable)
  - Builds all the vendored tools

Add dep instalation, remove govendor from go tools, and add a check for
dependancies.

Moves from goimports to [goreturns](https://github.com/sqs/goreturns).
This should not effect any code that already compiles as goreturns only
adds zero return values, then runs goimports.

This is mainly to help improve ergonomics when dealing with multiple
return types.