Wrap breakpoint flags into m64p_dbg_bkp_flags enum.

The unused conditional and counter flags have been removed to avoid any
confusion until those features are added.

An `M64P' prefix is also added to the flags. This matches the style of other
API flags.
Merge pull request #30 from wnayes/bpt-flag-enum

Wrap breakpoint flags into m64p_dbg_bkp_flags enum.