nix hash path, and preperatory refactors

- `nix store add` supports text hashing

  With functional test ensuring it matches `builtins.toFile`.

- Factored-out flags for both commands

- Move all common reusable flags to `libcmd`

  - They are not part of the *definition* of the CLI infra, just a usag
    of it.

  - The `libstore` flag couldn't go in `args.hh` in libutil anyways,
    would be awkward for it to live alone

- Shuffle around `Cmd*` hierarchy so flags for deprecated commands don't
  end up on the new ones

tests/functional/add.sh

@@ -45,3 +45,8 @@ clearStore
     [[ "$path1" == "$path2" ]]
     path4=$(nix store add --mode flat --hash-algo sha1 ./dummy)
 )
+(
+    path1=$(nix store add --mode text ./dummy)
+    path2=$(nix eval --impure --raw --expr 'builtins.toFile "dummy" (builtins.readFile ./dummy)')
+    [[ "$path1" == "$path2" ]]
+)