(docs) Clarify possible exit codes and behavior with negative exit codes on Windows
(PUP-2972) Remove += and -=

Previously the future parser supported += and -=. += was very similar to
the same operator that is in the current parser, but with a few
differences to make it consistent with the rest of the
language. The += operator no longer performed string concatenation and
instead acted like $a = $a + $b. The -= operator was added to complete
the set of operators available.

However, many people have been confused by these operators (even as they
exist in the current parser):

  * It looks like mutation, but the language does not have mutation
  * It looks like += from other languages, but doesn't behave like them
  * Shadowing a variable is something that needs to be explicit,
    otherwise it becomes confusing.

This removes += and -= from the future parser. They are still parsed,
but the validation step will issue errors if they are encountered.
(docs) Remove stale warning that Windows can't manage symlinks

Windows can manage symlinks, and we explain the limitations (extra permissions
needed, no-go on 2003) in the `ensure` attribute.
(PUP-2879) Add two nfsd-related upstart services to the blacklist
(PUP-2972) Improve error message for += and -=

The previous error message was very abrupt and didn't provide any kind
of guidance for the user. The new message for += and -= let's them know
that it is *no longer* supported and has a link to where they can get
more information.
Merge pull request #2958 from zaphod42/issue/master/pup-2972-remove-plussignment

(PUP-2972) Remove += and -=
Merge pull request #2959 from nfagerlund/docs_windows_symlink_straggler

(docs) Remove stale warning that Windows can't manage symlinks
Merge pull request #2960 from kylog/issue/pup-2879/add-more-services-to-blacklist

(PUP-2879) Add two nfsd-related upstart services to the blacklist
Merge pull request #2879 from nfagerlund/docs_exec_exitcodes_windows

(docs) Clarify possible exit codes and behavior with negative exit codes on Windows