Implement <sys/fsuid.h>.

(cherry picked from commit 79310994d2b3826a10598f7e7795acb5edb42a20)

Change-Id: I47688273691e5c95e5e9302eba254ccaaaad40ca
diff --git a/libc/SYSCALLS.TXT b/libc/SYSCALLS.TXT
index 014d554..128ec6b 100644
--- a/libc/SYSCALLS.TXT
+++ b/libc/SYSCALLS.TXT
@@ -303,6 +303,9 @@
 
 int __set_tid_address:set_tid_address(int*)  all
 
+int setfsgid(gid_t)  all
+int setfsuid(uid_t)  all
+
 pid_t wait4(pid_t, int*, int, struct rusage*)  all
 int __waitid:waitid(int, pid_t, struct siginfo_t*, int, void*)  all