Add DragonFly umtx_{sleep, wakeup}.

libc-test/semver/dragonfly.txt

@@ -1497,6 +1497,8 @@ timex
 truncate
 ttyname_r
 ucontext_t
+umtx_sleep
+umtx_wakeup
 unmount
 updatelastlogx
 updwtmpx

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

@@ -1481,6 +1481,9 @@ extern "C" {
     pub fn utmpxname(file: *const ::c_char) -> ::c_int;
 
     pub fn sys_checkpoint(tpe: ::c_int, fd: ::c_int, pid: ::pid_t, retval: ::c_int) -> ::c_int;
+
+    pub fn umtx_sleep(ptr: *const ::c_int, value: ::c_int, timeout: ::c_int) -> ::c_int;
+    pub fn umtx_wakeup(ptr: *const ::c_int, count: ::c_int) -> ::c_int;
 }
 
 #[link(name = "rt")]