userland ::whatis always complains about "umem_internal_arena" symbol
with a process target, ::whatis always complains:
mdb: unable to readvar "umem_internal_arena": unknown symbol name
The problem is that mdb_readvar() doesn't find symbols in shared libraries (bug #3651). In addition to fixing that bug, we should use umem_readvar(), which searches only libumem.so.