Optimize rdataset_getownercase not to use bitshifts

Optimize rdataset_getownercase not to use bitshifts

The last rdataset_getownercase() left it in a state where the code was
mix of microoptimizations (manual loop unrolling, complicated bitshifts)
with a code that would always rewrite the character even if it stayed
the same after transformation.

This commit makes sure that we modify only the characters that actually
need to change, removes the manual loop unrolling, and replaces the
weird bit arithmetics with a simple shift and bit-and.