Debugger: Fix #13797.

- When reading the areas note, there was no check in place to ensure that a
matching segment for the address was actually found. This would later lead
to a crash when attempting to read memory from the corresponding (missing)
piece of memory in the core.

Debugger: Split into core library and application.

- Add subfolder src/kits/debugger which contains the debugger's core
functionality and lower layers. Correspondingly add headers/private/debugger
for shared headers to be used by clients such as the Debugger application
and eventual remote_debug_server. Adjust various files to account for
differences as a result of the split and moves.
- Add libdebugger.so to minimal Jamfile.