lvmetad: set disabled flag when duplicate PVs are seen

lvmetad: set disabled flag when duplicate PVs are seen

When duplicate PVs are detected, set the disabled
flag so that commands will disable use of lvmetad.

This duplicate detection is done by lvmetad itself
when it's told about a single new PV with a PVID
that matches an existing PV on another device.
(This is different from the case where the command
is scanning all devices and detects the duplicate.)

Remove the "altdev" logic that attempted to keep
track of multiple devices for a single PV.  It
is no longer used since lvmetad is disabled in
this case.