The primary issue was the absense of a dssum when computing the new RHS.

Following the template of projh (navier4.F90), a mask and dssum
are applied to the output of axhelm on the coarse solve prior
to subtraction from the RHS.

This commit is converging slightly more quickly, wrt the number of
gmres steps, then with u = 0._dp, so its doing something.  There
is still room for bugs to be living in the kernel, though.