filters: check for mpath before opening devs

Split out the partitioned device filter that needs to open the device
and move the multipath filter in front of it.

When a device is multipathed, sending I/O to the underlying paths may
cause problems, the most obvious being I/O errors visible to lvm if a
path is down.

Revert the incorrect <backtrace> messages added when a device doesn't
pass a filter.

Log each filter initialisation to show sequence.

Avoid duplicate 'Using $device' debug messages.

lib/filters/filter-mpath.c

@@ -13,7 +13,7 @@
  */
 
 #include "lib.h"
-#include "filter-mpath.h"
+#include "filter.h"
 #include "activate.h"
 
 #ifdef linux
@@ -228,6 +228,8 @@ struct dev_filter *mpath_filter_create(struct dev_types *dt)
 	f->use_count = 0;
 	f->private = dt;
 
+	log_debug_devs("mpath filter initialised.");
+
 	return f;
 }