Auto merge of #2423 - rtzoeller:dragonfly_sched_getcpu, r=Amanieu

DragonFlyBSD supports sched_getcpu()

In addition to `sched_setaffinity()` and `sched_getaffinity()`, DragonFlyBSD also provides a way to check which CPU a thread is currently executing on.

src/unix/bsd/freebsdlike/dragonfly/mod.rs

@@ -1390,6 +1390,7 @@ extern "C" {
     pub fn sched_getaffinity(pid: ::pid_t, cpusetsize: ::size_t, mask: *mut cpu_set_t) -> ::c_int;
     pub fn sched_setaffinity(pid: ::pid_t, cpusetsize: ::size_t, mask: *const cpu_set_t)
         -> ::c_int;
+    pub fn sched_getcpu() -> ::c_int;
     pub fn setproctitle(fmt: *const ::c_char, ...);
 
     pub fn shmget(key: ::key_t, size: ::size_t, shmflg: ::c_int) -> ::c_int;