fd_util: add function dup_cloexec()

Unfortunately, there's no "optimized" implementation here.  We can't
use Linux's proprietary system call dup3(), because it would require
us to specify the new descriptor.

src/fd_util.c

@@ -103,6 +103,16 @@ fd_set_nonblock(int fd)
 #endif
 }
 
+int
+dup_cloexec(int oldfd)
+{
+	int newfd = dup(oldfd);
+	if (newfd >= 0)
+		fd_set_nonblock(newfd);
+
+	return newfd;
+}
+
 int
 open_cloexec(const char *path_fs, int flags, int mode)
 {