Add mono_counters_agent_stop and move fd to static variable to close it properly in mono_counters+agent_stop
Add call to mono_counters_agent_stop in countersagent profiler
mono counters agent : ensure that the file descriptor is closed before opening it again. it is usefull if we call mono_counters_agent_start multiple times without calling mono_counters_agent_stop, and we want to reset the output