plugin, daemon: Use '::' in plugin names to indicate subdirectories.

plugin, daemon: Use '::' in plugin names to indicate subdirectories.

That is, behave similar to Perl when encountering a '::' in a plugin's name and
translate that to a slash. "foo::bar" will thus become PKGLIBDIR/foo/bar.so.
This will be used to name plugins accessing the same backend using different
methods and to have an, imho, nicer name for those.

The daemon will now use that to prepend "backend::" to the name of backends.
This is no different to the behavior before but only uses different names that
will be displayed in log messages.