v2 encryption policies are now recommended, due to various security and
usability advantages over v1 policies.  Many people have been running
into the usability problems with v1, so it's desirable to get people
onto v2 without having to manually opt-in.

Therefore, when 'fscrypt setup' creates /etc/fscrypt.conf, enable
policy_version 2 automatically if the kernel supports it.

I decided to go with this solution over the policy_version "auto" I
suggested originally because this way is simpler, it can still be
changed to "auto" later if desired, and "auto" might require changing
how we parse the config file (since currently the config file is mapped
directly to a protobuf where policy_version is an 'int' and is shared
with EncryptionOptions).

Resolves https://github.com/google/fscrypt/issues/182

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Also removed an outdated sentence from the comment for setupBefore().

Addresses most of the issues in #101.

The following commands now succeed:
    GOARCH=mips go build github.com/google/fscrypt/util
    GOARCH=mipsle go build github.com/google/fscrypt/util

Now the testing functions will skip the integration tests if a testing
filesystem is not specified.