site setup: actually use calculated default for st->key_renegotiate_time

site setup: actually use calculated default for st->key_renegotiate_time

We go to some trouble to calculate an appropriate default value for
st->key_renegotiate_time.  However, when we actually do the config
file lookup we overwrote the result and used st->key_lifetime as the
default instead, which is wrong.

The upshot is that prior to this patch, DEFAULT_KEY_RENEGOTIATE_GAP
and the associated logic was unused, and keys were only renegotiated
at the point where they expired, which would produce a small gap in
connectivity.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>