add splice and vmsplice syscalls

based on patches by orc and Isaac Dunham.

src/linux/splice.c

@@ -0,0 +1,8 @@
+#define _GNU_SOURCE
+#include <fcntl.h>
+#include "syscall.h"
+
+ssize_t splice(int fd_in, off_t *off_in, int fd_out, off_t *off_out, size_t len, unsigned flags)
+{
+	return syscall(SYS_splice, fd_in, off_in, fd_out, off_out, len, flags);
+}

src/linux/vmsplice.c

@@ -0,0 +1,8 @@
+#define _GNU_SOURCE
+#include <fcntl.h>
+#include "syscall.h"
+
+ssize_t vmsplice(int fd, const struct iovec *iov, size_t cnt, unsigned flags)
+{
+	return syscall(SYS_vmsplice, fd, iov, cnt, flags);
+}