serve: drop "keys" strvec

serve: drop "keys" strvec

We collect the set of capabilities the client sends us in a strvec.
While this is usually small, there's no limit to the number of
capabilities the client can send us (e.g., they could just send us
"agent" pkt-lines over and over, and we'd keep adding them to the list).

Since all code has been converted away from using this list, let's get
rid of it. This avoids a potential attack where clients waste our
memory.

Note that we do have to replace it with a flag, because some of the
flush-packet logic checks whether we've seen any valid commands or keys.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>