implement semtimedop

this is a Linux-specific extension to the sysv semaphore api.

src/ipc/ipc.h

 #define IPCOP_semop      1
 #define IPCOP_semget     2
 #define IPCOP_semctl     3
+#define IPCOP_semtimedop 4
 #define IPCOP_msgsnd    11
 #define IPCOP_msgrcv    12
 #define IPCOP_msgget    13

src/ipc/semtimedop.c

+#define _GNU_SOURCE
+#include <sys/sem.h>
+#include "syscall.h"
+#include "ipc.h"
+
+int semtimedop(int id, struct sembuf *buf, size_t n, const struct timespec *ts)
+{
+#ifdef SYS_semop
+	return syscall(SYS_semtimedop, id, buf, n, ts);
+#else
+	return syscall(SYS_ipc, IPCOP_semtimedop, id, n, 0, buf, ts);
+#endif
+}