unix/mpthreadport: Switch back to use SIGUSR1 for thread-GC.
This signal was used from the beginning, and worked well. Peculiar
configurations should override it, not the other way around.
#include <sched.h>
#include <semaphore.h>
-#define MP_THREAD_GC_SIGNAL (SIGRTMIN + 5)
+#define MP_THREAD_GC_SIGNAL (SIGUSR1)
// this structure forms a linked list, one node per active thread
typedef struct _thread_t {
