MINOR: spoe: Move all stuff regarding the filter/applet in the C file

MINOR: spoe: Move all stuff regarding the filter/applet in the C file

Structures describing the SPOE applet context, the SPOE filter configuration
and context and the SPOE messages and groups are moved in the C file. In
spoe-t.h file, it remains the structure describing an SPOE agent and flags
used by both sides.

In addition, the SPOE frontend, created for a given SPOE engine, is moved
from the SPOE filter configuration to the SPOE agent structure.

The related issue is #2502.