Implement clock_adjtime syscall decoder

* linux/dummy.h (sys_clock_adjtime): Remove.
* linux/syscall.h (sys_clock_adjtime): New prototype.
* time.c (do_adjtimex): New function, based on sys_adjtimex.
(sys_adjtimex): Use it.
(sys_clock_adjtime): New function.
diff --git a/linux/syscall.h b/linux/syscall.h
index 5e61c11..7a66c84 100644
--- a/linux/syscall.h
+++ b/linux/syscall.h
@@ -44,6 +44,7 @@
 int sys_chdir();
 int sys_chmod();
 int sys_chown();
+int sys_clock_adjtime();
 int sys_clock_gettime();
 int sys_clock_nanosleep();
 int sys_clock_settime();