libutil: make Fd{Sink,Source} io buffer shareable

we will need this during RemoteStore wire asyncification to be able to
use the old synchronous serializers. alternatively we could define all
serializers on the async types as well, but that'd be slow and far too
much unnecessarily duplicated code (that will be deleted soon anyway).

Change-Id: I6e4f334025844b808a697ddcd8f80ddcd8c3fc9c

lix/libutil/serialise.hh

@@ -125,6 +125,7 @@ struct FdSink : BufferedSink
 
     FdSink() : fd(-1) { }
     FdSink(int fd) : fd(fd) { }
+    FdSink(int fd, ref<IoBuffer> buffer) : BufferedSink(buffer), fd(fd) {}
     FdSink(const FdSink &) = delete;
     FdSink(FdSink &&) = delete;
     FdSink & operator=(const FdSink &) = delete;
@@ -154,6 +155,7 @@ struct FdSource : BufferedSource
 
     FdSource() : fd(-1) { }
     FdSource(int fd) : fd(fd) { }
+    FdSource(int fd, ref<IoBuffer> buffer) : BufferedSource(buffer), fd(fd) {}
     FdSource(const FdSource &) = delete;
     FdSource(FdSource &&) = delete;
     FdSource & operator=(const FdSource &) = delete;