gdb: remove bpstat typedef, rename bpstats to bpstat

gdb: remove bpstat typedef, rename bpstats to bpstat

I don't find that the bpstat typedef, which hides a pointer, is
particularly useful.  In fact, it confused me many times, and I just see
it as something to remember that adds cognitive load.  Also, with C++,
we might want to be able to pass bpstats objects by const-reference, not
necessarily by pointer.

So, remove the bpstat typedef and rename struct bpstats to bpstat (since
it represents one bpstat, it makes sense that it is singular).

Change-Id: I52e763b6e54ee666a9e045785f686d37b4f5f849