As #19274 is now fixed in Go 1.9, there is no longer any reason to have
a duplicate implementation to the standard library. We can now move
safely to crypto/rand.

Replace IsUserRoot with CheckIfRoot. This allows all functions to use
the same error handling when a user is not root.

When looking up users in fscrypt, we often want to proceed even if the
requested uid doesn't appear to be a valid user on the system. This
mainly occurs when a user is deleted, but they still have a login
protector on disk.

Thus, GetUser() with a bad uid creates a fake user with a pretty
placeholder name. The corresponding call sites of util.EffectiveUser are
changed (often simplifying logic). Various documentation is updated and
typos are fixed.

Variables are generally declared at the top of a file.

Our .gitignore file was overly restrictive, fixed to only include the
fscrypt binary. Also, our build tags were incorrectly formatted. The
tags are removed and the corresponding lint rule is simplified.

The build tags will be added back after the refactor.

Typos

Makefile cleanup