Fix test so that it is /bin/sh compliant (issue #2742)

The old test used syntax that appeared to be bash-specific and did not
work on platforms where /bin/sh did not point to bash.  See issue #2742
where copy to solaris hosts failed with the error:

    output: {'stdout': '', 'stderr': '/bin/sh: test: argument expected\n',
             'rc': 1}

handlers must be run once

Documentation: Links are the best kinds of instructions

Nit fix spelling of separate

Link people right to this page on Github, so they can click "edit" right there. No need for them to search for the repo, figure out that the docsite isn't a separate repo, and navigate to the given dir.

sysctl check fails if a custom sysctl_file is given.

so remove all occurences of each host from the handlers notified by
lists

Don't send unicode instances to the selinux module

Showcase the new-style lookup plugin access in the authorized_key docs rather than the old-style $FILE

This fixes #2632.  Briefly: specifying things like paths using complex
args in a playbook will make the objects unicode instances.  The selinux
module does not accept unicode instances for its char * arguments; it
wants str instances.

Per mpdehaan's comment on #2632 I just went ahead and converted all
paths to UTF-8.  I don't know if it would be better to do something like
converting to locale.getpreferredencoding(), but I factored all the
conversions out into new method _to_filesystem_str, so there's only one
place that needs to be changed in the future.

Typo in example, extra space breaks playbook.

Showcase the new-style lookup plugin access in the authorized_key docs rather than the old-style $FILE