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)
 {

src/fd_util.h

@@ -49,6 +49,13 @@
 
 struct sockaddr;
 
+/**
+ * Wrapper for dup(), which sets the CLOEXEC flag on the new
+ * descriptor.
+ */
+int
+dup_cloexec(int oldfd);
+
 /**
  * Wrapper for open(), which sets the CLOEXEC flag (atomically if
  * supported by the OS).