Have control interface use less stack

[m-ronnblom]

The libxcm control interface (libxcm/ctl.c) uses ~38 kb stack, which may cause issues if the calling thread has a tiny stack configured.

It should be possible to peek on the control fd to see if data is available, and in that case do a 38 kb short-lived heap allocation. However this "peeking" should be efficient (as fast as recv() or faster), since the process_client() functions is run often (as long as a client is connected).