Additional MD component checking

If udev info is missing for a device, (which would indicate
if it's an MD component), then do an end-of-device read to
check if a PV is an MD component.  (This is skipped when
using hints since we already know devs in hints are good.)

A new config setting md_component_checks can be used to
disable the additional end-of-device MD checks, or to
always enable end-of-device MD checks.

When both hints and udev info are disabled/unavailable,
the end of PVs will now be scanned by default.  If md
devices with end-of-device superblocks are not being
used, the extra I/O overhead can be avoided by setting
md_component_checks="start".

lib/cache/lvmcache.c

@@ -555,8 +555,11 @@ next:
 	 */
 
 	if (!(info = lvmcache_info_from_pvid(alt->dev->pvid, NULL, 0))) {
-		/* This shouldn't happen */
-		log_warn("WARNING: PV %s on duplicate device %s not found in cache.",
+		/*
+		 * This will happen if a duplicate dev has been dropped already,
+		 * e.g. it was found to be an md component.
+		 */
+		log_debug("PVID %s on duplicate device %s not found in cache.",
 			 alt->dev->pvid, dev_name(alt->dev));
 		goto next;
 	}