add linux-specific unshare syscall wrapper

include/sched.h

@@ -56,6 +56,7 @@ int     sched_yield(void);
 #define CLONE_NEWNET	0x40000000
 #define CLONE_IO	0x80000000
 int clone (int (*)(void *), void *, int, void *, ...);
+int unshare(int);
 #endif
 
 #ifdef __cplusplus

src/linux/unshare.c

@@ -0,0 +1,8 @@
+#define _GNU_SOURCE
+#include <sched.h>
+#include "syscall.h"
+
+int unshare(int flags)
+{
+	return syscall(SYS_unshare, flags);
+}