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.

src/libmain/shared.hh

@@ -30,6 +30,10 @@ void printGCWarning();
 
 void printMissing(StoreAPI & store, const PathSet & paths);
 
+void printMissing(const PathSet & willBuild,
+    const PathSet & willSubstitute, const PathSet & unknown,
+    unsigned long long downloadSize, unsigned long long narSize);
+
 template<class N> N getIntArg(const string & opt,
     Strings::iterator & i, const Strings::iterator & end)
 {