building: Add comments describing access macros.
flag: Add accessor macros for flag struct fields.

Add macros FLAG_LAND_PATHS(), FLAG_IS_WATER_PATH(), FLAG_HAS_RESOURCES().
Add macros to check whether path has transporter: FLAG_TRANSPORTERS() and FLAG_HAS_TRANSPORTER().
Add macros FLAG_SERF_REQUEST_FAIL(), FLAG_SERF_REQUESTED().
Add macros for accessing transporter count: FLAG_TRANSPORTER_COUNT() and FLAG_LENGTH_CATEGORY().
Add macros for scheduled resources: FLAG_SCHEDULED_SLOT(), FLAG_IS_SCHEDULED().
Add macro FLAG_OTHER_END_DIR().
Add inventory related access macros: FLAG_HAS_INVENTORY(), FLAG_ACCEPTS_SERFS(), and FLAG_ACCEPTS_RESOURCES().
Add comments documenting flag accessor macros.