detect: unify functions for multi-buffer

detect: unify functions for multi-buffer

Ticket: 6575

Multi buffers keywords now use a single registration function
DetectAppLayerMultiRegister with a GetBuffer argument.

This GetBuffer function pointer is similar to the ones used by
single-buffer keyword, except that it takes an additional
parameter which is the index of the buffer to get.
Under the hood, an anonymous union between these 2 functions
pointers types is used.

In the end, this deduplicates code, especially the calls to
DetectEngineContentInspection