FAST_IPSEC bug fix

[Helge Oldach]

Mike Tancsa:
>Well, its not totally a bug, but missing functionality that looks      
>like is there but is not and is pretty important to keep lossy         
>links functioning with IPSEC. My colleague gabor at sentex.net created    
>the patch below that implements net.key.prefered_oldsa when using      
>FAST_IPSEC.                                                            

Yep, this is particularly important when running IPSec against other
vendors' IPSec implementation. Many appear to prefer the new SA over the
old one.

Actually this is the only issue that stopped me from going to
FAST_IPSEC.

Please also note that the nam of the sysctl has been changed in -CURRENT
about six weeks ago to net.key.preferred_oldsa (double "r"). I would
suggest to change it for RELENG_4 also, but *only* for FAST_IPSEC.

Helge