Added Security considerations

Added first draft of security considerations for discussion. Also,
uncommented-out a section in the IANA considerations that made xml2rfc
crazy due to double-hyphens in a comment.
Merged the security considerations text from Ben.
Some minor editorial corrections to the same new text.

Signed-off-by: Jouni Korhonen <jouni.korhonen@renesasmobile.com>