Add cwd_optional to command details and enable it in the sudoers plugin.

If cwd_optional is set to true, a failure to set the cwd will be a
warning, not an error, and the command will still run.
Debian bug #598519

src/sudo.h

@@ -157,6 +157,7 @@ struct command_details {
     int closefrom;
     int flags;
     int execfd;
+    int cwd_optional;
     struct preserved_fd_list preserved_fds;
     struct passwd *pw;
     GETGROUPS_T *groups;