Adjust the relaxation step to use the condition
```
abs(subdiag[i]) <= epsilon * sqrt(abs(diag[i]) + abs(diag[i+1]))
```
for setting the subdiagonal entry to zero.
Also adjust Wilkinson shift for small `e = subdiag[end-1]` -
I couldn't find a reference for the original, and it was not
consistent with the Wilkinson definition.
Fixes#2191.
90187a33e1