Updates/merge multiple policy

[r-scheele]

Description

This PR merges multiple policies in mc admin user policy TARGET USERNAME command, if more than one exists.

./mc admin user add myminio minio1 minio123
Added user `minio1` successfully.

$ ./mc admin policy list myminio
consoleAdmin
diagnostics
readonly
readwrite
writeonly

$ ./mc admin user policy myminio minio1
{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Effect": "Allow",
      "Action": [
        "admin:*"
      ]
    },
    {
      "Effect": "Allow",
      "Action": [
        "kms:*"
      ]
    },
    {
      "Effect": "Allow",
      "Action": [
        "s3:*"
      ],
      "Resource": [
        "arn:aws:s3:::*"
      ]
    }
  ]
}

[donatello]

I think output should indicate that this is a merged policy output of the policies applied to the user to reduce chances of confusion.

[harshavardhana]

In mc we treat go err as e and probe.Error as err

[harshavardhana]

PTAL @donatello

[harshavardhana]

PTAL @donatello @shtripat

[shtripat]

lgtm