ike-sa: Always set ike_cfg_t when setting peer_cfg_t

ike-sa: Always set ike_cfg_t when setting peer_cfg_t

This is more consistent and e.g. allows to properly take into account
some settings that are also relevant during IKE_AUTH (e.g. childless).

We also already use the peer_cfg_t's ike_cfg_t when rekeying,
reauthenticating and reestablishing an IKE_SA (and e.g. for DSCP).

Also changed are some IKEv1 cases where get_ike_cfg() is called before
set_peer_cfg() without taking a reference to the ike_cfg_t that might
get replaced/destroyed (none of the cases were problematic, though, but
it also wasn't necessary to keep the ike_cfg_t around).

Closes strongswan/strongswan#1238