On Linux, move the passing of filedescriptors to a dedicated socketpair().

sandbox/linux/seccomp/syscall.cc

Index: sandbox/linux/seccomp/syscall.cc
diff --git a/sandbox/linux/seccomp/syscall.cc b/sandbox/linux/seccomp/syscall.cc
index b25146b380c0bd1ee6cc134fbc9cf78013799f6e..8b14b302a4abc0a933621239ad186c715095d0ab 100644
--- a/sandbox/linux/seccomp/syscall.cc
+++ b/sandbox/linux/seccomp/syscall.cc
@@ -203,7 +203,7 @@ void* Sandbox::defaultSystemCallHandler(int syscallNum, void* arg0, void* arg1,
   // the exact instruction sequence in libc, we might not be able to reliably
   // filter out these system calls at the time when we instrument the code.
   SysCalls sys;
-  unsigned long rc;
+  long     rc;
   switch (syscallNum) {
     case __NR_read:
       Debug::syscall(syscallNum, "Allowing unrestricted system call");