Compile-time switch to enable 0.9 metadata as default

Compile-time switch to enable 0.9 metadata as default

This commit introduces DEFAULT_OLD_METADATA as a preprocessor
definition. If defined, it causes mdadm to assume metadata version 0.9
as default. If not defined, version 1.x (currently 1.2) is used as
default.

The man page mdadm.8 is also modified to reflect the chosen default.

The selftests will not work if the old default is chosen.

This patch was requested by Debian so they could distribute a current
mdadm together with boot loaders that only understand 0.90 metadata
for md-raid.

Preferred usage is simply
   make DEFAULT_OLD_METADATA=yes

Signed-off-by: martin f. krafft <madduck@debian.org>
Signed-off-by: NeilBrown <neilb@suse.de>