api: Add nbd_[aio_]opt_starttls

Very similar to the recent addition of nbd_opt_structured_reply, with
the new nbd_opt_starttls() finally giving us fine-grained control over
NBD_OPT_STARTTLS. and allowing productive handshaking with a server in
SELECTIVETLS mode.

With this patch, it is now easy to reproduce the scenario of nbdkit's
CVE-2021-3716, where a plaintext meddler-in-the-middle attacker could
cause client denial of service when a --tls=on server failed to reset
state after NBD_OPT_STARTTLS.  This also exposed the fact that nbdkit
was not gracefully handling NBD_OPT_INFO from a plaintext client
connecting to a --tls=require server; the testsuite is skipped unless
using a fixed nbdkit (v1.33.2 or later).