BUG/MEDIUM: ssl: Put CO_ER_SSL_KEYUPDATE at the right place

FOr some reason, CO_ER_SSL_KEYUPDATE has been put in the middle of the
enum containing all the different possible connection errors, but
reg-tests depend on their numeric value, so it broke them. Put it at the
end of the enum instead.

This should be backported up to 2.8 with commit
91004114fe8816f848025fe71def4ea23e72a5f6.