Don't send unicode instances to the selinux module

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.