Shortcut already-hardlinked files

If an inode in the Nix store has more than 1 link, it probably means that it was linked into .links/ by us. If so, skip.

There's a possibility that something else hardlinked the file, so it would be nice to be able to override this.

Also, by looking at the number of hardlinks for each of the files in .links/, you can get deduplication numbers and space savings.
Preload linked hashes to speed up lookups

By preloading all inodes in the /nix/store/.links directory, we can
quickly determine of a hardlinked file was already linked to the hashed
links.
This is tolerant of removing the .links directory, it will simply
recalculate all hashes in the store.
Use the inodes given by readdir directly
Shortcut store files before lstat

readdir() already returns the inode numbers, so we don't need to call
lstat to know if a file was already linked or not.
Merge branch 'master' of github.com:wmertens/nix
Remove tab
nix-store --optimise: Remove bogus statistics
lvlInfo -> lvlTalkative