Expose definitions relating to the CHERI trusted stack to userspace;
provide a variant of the stack frame that uses __capability pointers
rather than chericap for capability-aware userspace code.  Include a
comment on the rather imperfect ABI that this constitutes; we will
want to refine (fix) this later.
Export and import the whole CHERI trusted stack, not just the trusted-stack
pointer, for the CHERI_GET_STACK and CHERI_SET_STACK sysarch()s.
Implement cheritest_failure_err(), used by /bin/cheritest in situations
similar to those in which err() would be used to print errno messages
on test failures.
Add a test case for the CHERI_GET_STACK sysarch(), which queries the trusted
stack from within the system class as invoked from a sandbox and checks that
both the ambient and system code cspabilities are present as expected.

This required some general cleanup of the cheritest-helper class so that it
can pass throgpugh arbitrary system-class method numbers, as we use
system-class extensions to call back into thr test framework from sandboxes.

Pleasingly, the test passes nicely, so querying the trusted stack from
userspace appears to work well.