Logarithmic quantities should support some ufuncs

Currently,
```
lq = [2,3,4,5]*u.STmag
lq - lq
#
np.subtract(lq, lq)
# UnitConversionError: Can only apply 'subtract' function to quantities with compatible dimensions
np.minimum(lq, lq)
# UnitConversionError: Can only apply 'minimum' function to quantities with compatible dimensions
```
A combination of the helper functions not working for logarithmic units, and the wrong helper functions being used.

Reproduce the documented examples with logarithmic quantities and compare operator subtraction with np.subtract and np.minimum. Inspect Astropy's logarithmic-unit ufunc helper functions and determine which helpers are used for these operations; done means the NumPy ufunc calls produce compatible results instead of UnitConversionError.

Written by the indexing model from the issue text.