Add a group_by action plugin.

Use return codes > 255 for Ansible-specific errors.

More useful error message for if not HAVE_SELINUX

I had made and pushed this change after you already pulled the request.

@dhozac indicated that it would probably be better to use return codes > 255 for anything related to Ansible itself. Which makes sens :)

add check that vars plugins are not handing back None

Missing errors-import causes traceback

and the next one gets ignored despite being installed.

also clean up unnecessary second check.

Send a host to runner executor instead of a letter.

Discovered in #1463 .

Don't add file module arguments by default

This allows the postgresql_db module to work properly, as it uses
an argument named owner that conflicts with the file module.

Use previous proven multiprocessing logic as the simplification didn't have the same Ctrl-C handling and may
be subject to race issues, though still don't pass Runner to each.  Still seems performant.

Initialize sftp variable to fix error on transferless session

Make sure we always return 'rc' from the command module

If this is not a certainty, playbooks will fail without an 'rc' and checking both if there is an rc, and whether the 'rc' is (not) 0 is very complicated. (especially because ${something.rc} will not be substituted and all that)

Explicitly execute /bin/sh versus $SHELL because some folks have very incompatible/weird shells.  We do not
execute modules through the shell, this is only for basic ops, so should be sufficient.