gdb/dap - Getting thread names

gdb/dap - Getting thread names

Renamed thread_name according to convention (_ first)

When testing firefox tests, it is apparent that
_get_threads returns threads with name field = None.

I had initially thought that this was due to Firefox setting the names
using /proc/pid/task/tid/comm, by writing directly to the proc fs the
names, but apparently GDB seems to catch this, because I re-wrote
the basic-dap.exp/c to do this specifically and it saw the changes.

So I couldn't determine right now, what operation of name change that
GDB does not pick up, but with this patch, GDB will pick up the thread
names for an applications that set the name of a thread in ways that
aren't obvious.