Add location independent flag and functions to ignore mdas.

Add location independent flag and functions to ignore mdas.

First we add a 'flags' field to the location independent
metadata_area structure, and a MDA_IGNORE flag.  The
mda_is_ignored and mda_set_ignored functions are added to
manage the flag.  Adding the flag and functions gives a
library interface to ignore metadata areas independent of
the underlying location (disk, file, etc).  The location
specific read/write functions must then handle the specifics
of what this flag means to the location.

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