site setup: move st->key_renegotiate_time default calculation nearer use

st->key_renegotiate_time is first calculated as a default and then
that value ought to be used as a default for the config lookup.  So
move the default calculation near where the config lookup takes place
to keep the related code together.

Pure code motion; no functional change.  Also, the value computed is
not currently used due to a bug which will be fixed in the next patch.

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