Add -r option to darcs-all, and remove push-all (#3375)

Add -r option to darcs-all, and remove push-all (#3375)
Contributed by: seliopou@gmail.com

This patch modifies darcs-all to have feature parity with push-all by
recognizing two new options.

    * -i, equivalent to --ignore-failure in push-all
    * -r <repo>, specifies the remote repository darcs commands will use

Some example commands:

Get the libraries from a repository of your choosing. This is useful
when working with a git mirror:

    $ ./darcs-all -r http://darcs.haskell.org get

Pull changes. Used to be:

    $ ./push-all --pull http://darcs.haskell.org

Is now:

    $ ./darcs-all -r http://darcs.haskell.org pull

Or to use the default remote of the ghc repository:

    $ ./darcs-all pull