Additional MD component checking

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".