Previously, drgn had no way to represent a thread – retrieving a stack
trace (the only extant thread-specific operation) was achieved by
requiring the user to directly provide a tid.
This commit introduces the scaffolding for the design outlined in
issue #92, and implements the corresponding methods for userspace core
dumps, the live Linux kernel, and Linux kernel core dumps. Future work
will build on top of this commit to support live userspace processes.
Signed-off-by: Kevin Svetlitski <svetlitski@fb.com>
