Close a race during protocol startup.

Specifically, if the timeline looks like
	A sends magic and version
	B sends magic and version
	B receives magic and version
	B decides to continue
	B writes confirmer byte
	A receives magic, version, and confirmer
then A can find itself with the confirmer alread read from the network
when it enters EP_R_AWAIT_W.  The previous code abort()ed in this
circumstance.

Now, ep_read_step makes ep_rd_process defer further processing upon
entering EP_R_AWAIT_W.