[consfigurator] v2 pass data source

[David Bremner]

Hopefully I caught most of Sean's comments from the previous version.  This
version impliments much stricter validation for IDEN1 and IDEN2 (btw it would
be good to have a name for these). Along the way it introduces
#'valid-hostname-p, and extends the test suite to set up and tear down a gpg
and pass setup to test against.

I thought about making the new #'literal-data-pathname handle a list of
strings (or whatevers) but this (data source specific) API seems a bit nicer
to me.