Refactor onion_generate_cpath to build cpaths one hop at a time. This

Refactor onion_generate_cpath to build cpaths one hop at a time.  This
is a the first step in computing hops one step at a time.  Next, we move
the responsibility for calling onion_extend_cpath into circuit.c

(Later, we may want to special-case onion_extend_cpath to treat entry
and exit routers differently.)

svn:r792