Allow selection (and ordering) of column names

Build the creds table with only the columns we select,
in the order selected.

Example:

creds -s ssh -u root -S pubk -c port,user,proof

Credentials
===========

port  user  proof
----  ----  -----
22    root  KEY=30:...
22    root  KEY=37:...
22    root  KEY=30:...
22    root  KEY=37:...
22    root  KEY=a6:...
22    root  KEY=30:...
22    root  KEY=37:...
22    root  KEY=a6:...
22    root  KEY=30:...
22    root  KEY=37:...
22    root  KEY=a6:...
22    root  KEY=30:...
22    root  KEY=37:...
22    root  KEY=a6:...
22    root  KEY=30:...
22    root  KEY=37:...
22    root  KEY=30:...
22    root  KEY=37:...
22    root  KEY=a6:...
22    root  KEY=30:...
22    root  KEY=37:...