MAJOR: spoe: refactor the filter to clean up the code

The SPOE code is now pretty big and it was the good time to clean it up. It is
not perfect, some parts remains a bit ugly. But it is far better now.
diff --git a/include/proto/checks.h b/include/proto/checks.h
index 3c6eb7f..eb26c9a 100644
--- a/include/proto/checks.h
+++ b/include/proto/checks.h
@@ -52,8 +52,8 @@
 int srv_check_healthcheck_port(struct check *chk);
 
 /* Declared here, but the definitions are in flt_spoe.c */
-int prepare_spoe_healthcheck_request(char **req, int *len);
-int handle_spoe_healthcheck_response(char *frame, size_t size, char *err, int errlen);
+int spoe_prepare_healthcheck_request(char **req, int *len);
+int spoe_handle_healthcheck_response(char *frame, size_t size, char *err, int errlen);
 
 #endif /* _PROTO_CHECKS_H */