Rework key_newsav().

This function used by SADB_GETSPI and SADB_ADD handlers. It allocates new
SA and depending from SADB message does some initialization.
SADB_GETSPI initializes only few SA's fields. This SA stored with LARVAL
state in savtree_larval list of SA head. Then it can be updated by
SADB_UPDATE message.
SADB_ADD initializes all SA's fields and this SA stored with MATURE state
in savtree_alive list, after initialization it is allowed to change only
SA's lifetime fields.