Rework key_update().

Use SADB_CHECKHDR/SADB_CHECKLEN macros and key_checksockaddrs() to do
sanity checks. From now SADB_UPDATE for MATURE SAs only can update SA
state and lifetimes, for LARVAL SAs all fields except src+dst+SPI can
be updated. This follows RFC and allows to do access to almost all
SA fields without holding a lock.