Fix check for presence of ECDSA public key for outgoing connections.

At this point, c->config_tree may or may not be NULL, but this does not tell us whether it is an
outgoing connection or not. For incoming connections, we do not know the peer's name yet,
so we always have to claim ECDSA support. For outgoing connections, we always need to check
whether we have the peer's ECDSA public key, so that if we don't, we correctly tell the peer that
we want to upgrade.
Use read_host_config() where appropriate.
Don't free ephemeral ECDH keys twice.

ecdh_compute_shared() was changed to immediately delete the ephemeral key after
the shared secret was computed.  Therefore, the pointer to the ecdh_t struct
should be zeroed so it won't be freed again when a struct sptps_t is freed.