The FP_ identifiers referred to fingerprints, but they also applied
to address ranges.  The router_status_t name invited confusion with
routerstasus_t.  Fixes ticket 29826.

This is an automated commit, generated by this command:

./scripts/maint/rename_c_identifier.py \
        router_status_t rtr_flags_t \
        FP_INVALID RTR_INVALID \
        FP_BADEXIT RTR_BADEXIT \
        FP_REJECT RTR_REJECT