Added functions to register callbacks to the param_sniffer()

[kivkiv12345]

This is your chance to stop me @troelsjessen! :)

There's still a couple of things I want to change.
But if you have any major grievances with the general concept, you can let me know before I continue.

I've tested it ad-hoc, with the following function/callbacks:

void my_sniffer_callback(param_t * param, void *context) {
    printf("AAA %s %s\n", param->name, (char*)context);
}

void my_other_sniffer_callback(param_t * param, void *context) {
    printf("BBB %s%s\n", param->name, (char*)context);
}

void register_callbacks(void) {
    param_sniffer_register_callback(my_sniffer_callback, "hello there");
    param_sniffer_register_callback(my_other_sniffer_callback, ". What is up?");
}

void unregister_callbacks(void) {
    param_sniffer_unregister_callback(my_sniffer_callback);
    param_sniffer_unregister_callback(my_other_sniffer_callback);
}

pull after calling register_callbacks():

BBB csp_buf_out. What is up?
AAA csp_buf_out hello there
BBB csp_conn_out. What is up?
AAA csp_conn_out hello there
BBB csp_conn_ovf. What is up?
AAA csp_conn_ovf hello there
BBB csp_conn_noroute. What is up?
  51:124  csp_buf_out          = AAA csp_conn_noroute hello there
0                   
  52:124  csp_conn_out         = 0                   
BBB csp_inval_reply. What is up?
...
AAA csp_print_rdp hello there
BBB csp_print_packet. What is up?
AAA csp_print_packet hello there

Pull after calling unregister_callbacks():

  51:124  csp_buf_out          = 0                   
  52:124  csp_conn_out         = 0                   
...
  58:124  csp_print_rdp        = 0                   
  59:124  csp_print_packet     = 0                   
 1001:124  test_array_param     = [0 1 2 3 4 5 6 7]   
 1002:124  test_str             = 

Valgrind tested, no errors.

[jeanbaptistelab]

If we "probably should", why not do it ? :)

[kivkiv12345]

I take it you agree then? I will do it!

[jeanbaptistelab]

What about that ? This is a way for users of this API to register things, we can't be responsible for what they actually register, can we ?

[kivkiv12345]

I left that comment wondering how I should handle it in param_sniffer_UNregister_callback(). Do we unregister all callbacks to that function, or only the first match?
Do we error if that function is already registered?

[jeanbaptistelab]

A solution would be to update the
int param_sniffer_unregister_callback(param_sniffer_callback_f callback)
to:
int param_sniffer_unregister_callback(param_sniffer_callback_f callback, void * context)
and then test for both the callbackand contextto find the correct element to unregister...

[kivkiv12345]

@jeanbaptistelab Maybe it's overkill to hide SLIST? Especially since I've been considering exposing the callback collection itself as well. But I think that can be a future addition :)