Change struct breakpoint to m64p_breakpoint.

The breakpoint struct name was inconsistent with the rest of the `m64p' prefixed
declarations.

The API DebugBreakpointCommand was also changed to specify a m64p_breakpoint
pointer instead of a void pointer. There are no uses for the pointer other than
for referencing a breakpoint.
Merge pull request #29 from wnayes/bpt-api-type

Change struct breakpoint to m64p_breakpoint.