userspace: introduce default memory domain

We make a policy change here: all threads are members of a
memory domain, never NULL. We introduce a default memory domain
for threads that haven't been assigned to or inherited another one.

Primary motivation for this change is better MMU support, as
one common configuration will be to maintain page tables at
the memory domain level.

Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>