Add mda location specific mda_copy constructor.

Add mda location specific mda_copy constructor.

Because of the way mdas are handled internally, where a PV in a VG
has mdas on both info->mdas and vg->fid->metadata_areas list, we
need a location independent copy constructor for struct
metadata_area.  Break up the existing format-text specific copy
constructor into a format independent piece and a format dependent
piece.

This function is necessary to properly implement pv_set_mda_ignored().

Signed-off-by: Dave Wysochanski <dwysocha@redhat.com>
Reviewed-by: Alasdair G Kergon <agk@redhat.com>