Add security checks before using temp files for SELinux RBAC sudoedit.

Otherwise, it may be possible for the user running sudoedit to
replace the newly-created temporary files with a symbolic link and
have sudoedit set the owner of an arbitrary file.
Problem reported by Matthias Gerstner of SUSE.

src/sudo_exec.h

@@ -1,7 +1,7 @@
 /*
  * SPDX-License-Identifier: ISC
  *
- * Copyright (c) 2010-2016 Todd C. Miller <Todd.Miller@sudo.ws>
+ * Copyright (c) 2010-2017, 2020-2021 Todd C. Miller <Todd.Miller@sudo.ws>
  *
  * Permission to use, copy, modify, and distribute this software for any
  * purpose with or without fee is hereby granted, provided that the above
@@ -84,9 +84,11 @@
  */
 struct command_details;
 struct command_status;
+struct stat;
 
 /* copy_file.c */
 int sudo_copy_file(const char *src, int src_fd, off_t src_len, const char *dst, int dst_fd, off_t dst_len);
+bool sudo_check_temp_file(int tfd, const char *tname, uid_t uid, struct stat *sb);
 
 /* exec.c */
 void exec_cmnd(struct command_details *details, int errfd);