Detail: ensure --export names are acceptable as shell variables.

Detail: ensure --export names are acceptable as shell variables.

If an array contains a device which has a name that
contains something other than alphnumerics and underscores,
then some values reported by "mdadm --detail --export" will
not be valid as variable assignment of the shell.
This particularly affects dm devices.
e.g.
   MD_DEVICE_dm-4_ROLE=1
   MD_DEVICE_dm-4_DEV=/dev/dm-4

As it is particularly useful to be able to work with these
in a shell script, and as the precise name is not important,
change all non-alphanumerics to '_'.

   MD_DEVICE_dm_4_ROLE=1
   MD_DEVICE_dm_4_DEV=/dev/dm-4

Signed-off-by: NeilBrown <neilb@suse.com>
Signed-off-by: Jes Sorensen <jsorensen@fb.com>