metadata: Use flags to control warnings.

metadata: Use flags to control warnings.

The warnings arg was used to enable logging of warnings
when reading a PV.  This arg is turned into a set of flags
with the WARN_PV_READ flag matching the existing behavior.

A new flag WARN_INCONSISTENT is added that will cause
vg_read_internal() to log the "VG is not consistent"
warning so the various callers do not need to log
this warning themselves.

A new vg_read flag READ_WARN_INCONSISTENT is used from
reporting to enable the WARN_INCONSISTENT flag in
vg_read_internal.

[Committed by agk with cosmetic changes and tweaks.]