Kill the types xbt_context_function_t, smx_process_code_t and m_process_code_t; use...

Kill the types xbt_context_function_t, smx_process_code_t and m_process_code_t; use a uniq xbt_main_func_t defined in include/src/function_types.h instead. This reduces the amount of supurious casts around, and prevent me from introducing yet another 'gras_process_code_t' alias ;) This symbol was part of the MSG public interface, but I doubt any user ever created a variable of that type (this is a pointer to function)

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/simgrid/simgrid/trunk@3719 48e7efb5-ca39-0410-a469-dd3cf9ba447f