Micro AIs: split up Guardian MAI engine file

... into a separate file for each animal AI. This is a first step
toward separating the guardian MAIs, as keeping them together is not
needed any more.
Guardian MAIs: get rid of guardian_type= key

The previous value of guardian_type= now goes directly into ai_type=
Guardian MAIs: add syntax translation and deprecation message

So that the ai_type=guardian_unit guardian_type=... syntax still works
for the time being.
Guardian MAIs: add individual engine macros for each AI

... so that only the code for the guardian AI used is loaded.
Add deprecation message for MICRO_AI_GUARDIAN macro
Guardian MAIs: rename internal engine functions and CAs

This shouldn't affect anything else, it is simply done for consistency
(to reflect the recent syntax changes).