Allow regular files as GC roots

If a root is a regular file, then its name must denote a store
path. For instance, the existence of the file

  /nix/var/nix/gcroots/per-user/eelco/hydra-roots/wzc3cy1wwwd6d0dgxpa77ijr1yp50s6v-libxml2-2.7.7

would cause

  /nix/store/wzc3cy1wwwd6d0dgxpa77ijr1yp50s6v-libxml2-2.7.7

to be a root.

This is useful because it involves less I/O (no need for a readlink()
call) and takes up less disk space (the symlink target typically takes
up a full disk block, while directory entries are packed more
efficiently). This is particularly important for hydra.nixos.org,
which has hundreds of thousands of roots, and where reading the roots
can take 25 minutes.
Make readDirectory() return inode / file type
findRoots(): Prevent a call to lstat()

This means that getting the roots from /nix/var/nix/.../hydra-roots
doesn't need any I/O other than reading the directory.