Clarify selection of MAC, elaborate on MAC chaining and path splicing

[matzf]

Describe AES-CMAC hop field MAC algorithm as the default which MUST be supported.

• Moved the details of the default mac algorithm to after the explanation of the accumulator field (as this is used in the mac input).
• Fixed the swapped info field/hop field labels in the figure depicting the MAC input.
• Describe requriements on alternative MAC implementations, in particular about requirements for MAC chaining
• Describe path splicing is prevented
• Describe how path splicing can still happen in security considerations

Contributes to https://github.com/scionassociation/scion-cp_I-D/issues/23. Contributes to https://github.com/scionassociation/scion-dp_I-D/issues/7