Add sem_timedwait for unix platforms

We can almost implement synchronization mechanisms in pure-rust using
pthread semaphores as the primitive but sem_timedwait is necessary for
implementing timeout-based functions.

src/unix/mod.rs

@@ -710,6 +710,8 @@ extern {
                link_name = "sem_wait$UNIX2003")]
     pub fn sem_wait(sem: *mut sem_t) -> ::c_int;
     pub fn sem_trywait(sem: *mut sem_t) -> ::c_int;
+    pub fn sem_timedwait(sem: *mut sem_t,
+                         abstime: *const ::timespec) -> ::c_int;
     pub fn sem_post(sem: *mut sem_t) -> ::c_int;
     pub fn sem_init(sem: *mut sem_t,
                     pshared: ::c_int,