libflux: rename route enable/disable etc

Rename FLUX_MSGFLAG_RTE to FLUX_MSGFLAG_ROUTE
Rename flux_msg_set_rte (zmsg, true) to flux_msg_enable_route(zmsg)
Rename flux_msg_set_rte (zmsg, false) to flux_msg_clear_route(zmsg)
Eliminate flux_msg_get_rte.

Ensure that route accesors do the right thing with an empty
route stack (set id to NULL) and with a message that has no route
delim (EPROTO).

Add inline comments for routing functions in message.h and update
unit tests.
libflux: request.c update use of renamed route functions
api: update use of renamed route functions
broker: update use of renamed route functions
libflux: inline doc comment on payload functions