dag: use buffered I/O in FileNode.Signature()
dag: use buffered I/O in FileNode.Signature()

Now it takes two system calls to read a 200-byte file, not 201 (the
extra one reads the empty string and detects EOF).

Also: fix file descriptor leak.

sbinet/fubsy,sbinet/fubsy,sbinet/fubsy,sbinet/fubsy,sbinet/fubsy