Determine whether door_return actually drops anything

[robertdfrench]

The definition of door_return given in src/illumos/door_h.rs indicates that door_return never returns to the calling thread. This should be enough to persuade rustc to Drop anything that needs dropping, right? Or wrong?

[robertdfrench]

This will help figure out whether the demo applications encourage people to leak memeory

[robertdfrench]

Does not seem to drop everything -- there seems to be a constant amount of memory leaked per door invocation in the examples. I think that the buffers passed to door_return are allocated anew on each door_call, and then never deallocated. Need to confirm this, but if it's the case, then might want to have a reserved buffer for "return data", or some way (egads) deallocating the previously used buffer on subsequent door invocations.