ofproto: Simplify thread creation API.

ofproto: Simplify thread creation API.

There's no particular reason for the function controlling the number
of threads to be bound up with dpif_recv_set().  This patch breaks
them up, but as a side effect means threads will run doing nothing
when datapath upcall receiving is disabled.  By doing this, the udpif
thread creation API becomes a bit easier to reason about once there
are multiple types of thread introduced in future patches.

Signed-off-by: Ethan Jackson <ethan@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>