nix-store -r: Add ‘--ignore-unknown’ flag

This flag causes paths that do not have a known substitute to be
quietly ignored.  This is mostly useful for Charon, allowing it to
speed up deployment by letting a machine use substitutes for all
substitutable paths, instead of uploading them.  The latter is
frequently faster, e.g. if the target machine has a fast Internet
connection while the source machine is on a slow ADSL line.