Various bug fixes and cleanups to STP library.

Various bug fixes and cleanups to STP library.

    - Don't apply endian conversions to flags, which are 8 bits.
    - Use #defines for default times for use outside library.
    - Clarify our behavior when in STP_DISABLED state.
    - Add "aux" member to STP port struct to be able to refer back to
      the owning port.
    - Define macros to print STP bridge and port ids.
    - New helper function to get port id.
    - New helper function to convert speed to cost.
    - New functions to describe current role of port.