btrace: Transfer ownership of pointers.

btrace: Transfer ownership of pointers.

Directly insert new btrace_function pointers into the vector and have the
vector own these pointers.  This allows us to later retrieve these objects by
their number directly after creation whereas at the moment we have to wait
until the vector is fully populated.

This requires to pull btrace_thread_info through different functions but
cleans up the code for freeing the trace.