Add 'restart' arg to various functions used for reshaping.

Add 'restart' arg to various functions used for reshaping.

When we restart an array in the middle of a reshape, we reuse a lot of
the code for starting the reshape, but it needs to know that
circumstances are slightly different.

So add a 'restart' arg which is used:
 - skip checking and adding spares
 - activate the array (rather than start reshape)
 - allow the backup file to already exist

Signed-off-by: NeilBrown <neilb@suse.de>