Update comments about IVs used in IPsec ESP.

Add some prose and a diagram describing the layout of the cipher IV
for AES-CTR and AES-GCM and how it relates to the ESP IV stored in the
packet after the ESP header. Also, remove an XXX comment about the
initial block counter value used for AES-CTR in esp_output as the
current code matches the RFC (and the equivalent code in esp_input
didn't have the XXX comment).

Discussed with: cem