minor rpc interface cleanup; man pages with tested examples; hooks for extending flux_rpc_t

[garlick]

Changed flux_rpc_t typedef to refer to an incomplete struct definition, not a pointer to one. All users updated.

Bug fix: stop the message watcher for an RPC continuation once the RPC is complete.

Add man pages for new RPC usage including examples. Examples are compiled and run as part of sharness tests to make sure they don't fall out of date.

Add some functions to facilitate extending/wrapping flux_rpc_t as discussed in issue #76

const char *flux_rpc_type_get (flux_rpc_t *rpc);
void flux_rpc_type_set (flux_rpc_t *rpc, const char *type);
void *flux_rpc_aux_get (flux_rpc_t *rpc);
void flux_rpc_aux_set (flux_rpc_t *rpc, void *aux, flux_free_f destroy);

[grondo]

Nice touch with the examples that are compiled as part of testing. We should encourage that practice for all examples in documentation.

Looks like recent merge of your other PR means this one needs to be rebased.