add a client API for internally-triggered detach from dstack

Today our detach has some limitations: it can only be triggered from a
non-DR stack: an app stack via the start/stop API, or an injected thread
stack via a nudge on Windows

This issue covers a feature where a client can call a new APi routine
dr_detach_process() from the dstack. We could do a raw mmap to make a
stack, copy the current thread's app state there, switch to that stack,
call detach_on_permanent_stack(), and then swap to the stored state. We'd
just leak the mmap I guess. And no support for unloading the DR lib.