- Use define-derived-mode to declare nix-mode
- Use autoloads to ensure nix-mode is usable (and enabled) without needing `require`
- Use set + make-local-variable instead of longer 2-step equivalent

Signed-off-by: Ricky Elrod <ricky@elrod.me>

http://hydra.nixos.org/build/10170940

In Nixpkgs, the attribute in all-packages.nix corresponding to a
package is usually equal to the package name.  However, this doesn't
work if the package contains a dash, which is fairly common.  The
convention is to replace the dash with an underscore (e.g. "dbus-lib"
becomes "dbus_glib"), but that's annoying.  So now dashes are valid in
variable / attribute names, allowing you to write:

  dbus-glib = callPackage ../development/libraries/dbus-glib { };

and

  buildInputs = [ dbus-glib ];

Since we don't have a negation or subtraction operation in Nix, this
is unambiguous.

fixes to nix-worker systemd service descriptor: - remove commented-out lines - register the file for distribution in Makefile.am

    x.y.z or default

  (as originally proposed in
  https://mail.cs.uu.nl/pipermail/nix-dev/2009-September/002989.html).

  For instance, an expression like

    stdenv.lib.attrByPath ["features" "ckSched"] false args

  can now be written as

    args.features.ckSched or false

this enables (require 'nix-mode)

  much as possible.  (This is similar to GNU Make's `-k' flag.)

* Refactoring to implement this: previously we just bombed out when
  a build failed, but now we have to clean up.  In particular this
  means that goals must be freed quickly --- they shouldn't hang
  around until the worker exits.  So the worker now maintains weak
  pointers in order not to prevent garbage collection.

* Documented the `-k' and `-j' flags.