Replace all sprintf with safer snprintf

[seanm]

Even after https://github.com/HDFGroup/hdf5/pull/1815 there are 16 remaining uses of HDsprintf() left. They were a bit too tricky for me to confidently change as I don't know the codebase well.

[seanm]

@jhendersonHDF

[seanm]

I took another stab at this, and was disappointed to see there are more sprintf than there used to be.

Anyway, I fixed more of them. After this there is only 1 left!

[jhendersonHDF]

Thanks @seanm, I'd forgotten about this; will review.

[derobins]

All done. Yay!