Fix two issues in our backtrace code:

use "nm -an", not "nm -agn" which supresses local symbols.

Let backtrace_symbols() handle anything past _end.
Unhide the functions we emit, so they appear in a backtrace.
Don't hand objects over to EXP until we have fetched the body.

This closes (some of) a race where EXP would start to dismantle
an object while it was still being streamed.

Also spotted by: jw
Add an assert for clarity.
Add more paranoia.
Snapshot obj->st->len so it staus consistent in the loop.
Update st->len inside the bo->mtx in VBO_extend() so that it
is always synchronized with obj->len.