add linux setfs[ug]id syscall wrappers

patch by Jeremy Huntwork

src/linux/setfsgid.c

@@ -0,0 +1,8 @@
+#include <unistd.h>
+#include "syscall.h"
+#include "libc.h"
+
+int setfsgid(gid_t gid)
+{
+	return syscall(SYS_setfsgid, gid);
+}

src/linux/setfsuid.c

@@ -0,0 +1,8 @@
+#include <unistd.h>
+#include "syscall.h"
+#include "libc.h"
+
+int setfsuid(uid_t uid)
+{
+	return syscall(SYS_setfsuid, uid);
+}