Make sure that if --assemble find an array in the critical region
of a reshape, and cannot find the critical data to restart the
reshape, it gives an error message.

We were multiplying MD_RESERVED_SECTORS by 1024 instead of 512,
so lots of 'is it too small' checks were wrong.

When adding new disk to an array, don't reserve so much bitmap
space that the disk cannot store the required data. (Needed when
1.x array was created with older mdadm).

From: Ian Dall <ian@beware.dropbear.id.au>

I have a small patch to mdadm which allows the write-behind amount to be
set a array grow time (instead of currently only at grow or create
time). I have tested this fairly extensively on some arrays built out of
loop back devices, and once on a real live array.

Missing devices as well as failed devices cause an error.

This is equivalent to raidautorun that some distros provide.