client: Fix obscure readline hang

client: Fix obscure readline hang

If recv_input() can't stuff the whole line into @inbuf, it leaves its
tail in @input_from_rl.  If send_input() then empties @inbuf, the next
iteration will select @input_fd for reading instead of @sock for
writing, because @inbuf is empty.  Since @has_rl_input is still set,
recv_input() will do nothing, and the client hangs.

Fix as follows.  Factor ring_from_rl() out of recv_input().  Also call
it in send_input() to refill @inbuf from @input_from_rl.

Signed-off-by: Markus Armbruster <armbru@pond.sub.org>