Add make devcheck for developers
    
    Ticket 6604 makes pylint and jsl optional dependencies. The change
    is controversal, because some developers prefer that pylint and jsl
    should be required unless explicitly disabled.
    
    `make devcheck` is my answer to address the concerns. It's a superior
    solution to `make lint` as pre-commit check. It combines several
    additional checks under a single, easy rememberable and convenient make
    target:
    
    * build all
    * acilint, apiclient, jslint, polint
    * make check
    * pylint under Python 2 and 3
    * subset of unit test suite
    
    https://fedorahosted.org/freeipa/ticket/6604
    
    Signed-off-by: Christian Heimes <cheimes@redhat.com>
    Reviewed-By: Stanislav Laznicka <slaznick@redhat.com>