Pull up following revision(s) (requested by knakahara in ticket #840):

sys/net/if_ipsec.c: revision 1.15,1.16

Fix panic when ipsecif(4) adds discard policy. Pointed out by ohishi@IIJ, thanks.
Reviewd by ohishi@IIJ. Sorry, I jumped the gun and committed.

Fix the following two issues.
    - remove extra padding of sizeof(xisr) when adding ipsec policy
    - add padding for xpl when adding discard policy

(martin)