feat: extend file list PDU to transfer UNIX PERM

1. used 4 bytes out of a reserved 16 bytes section to store perm u32

2. add FLAGS_FD_UNIX_MODE: u32 = 0x08, used with flags, indicating this
   message is from UNIX peer

Signed-off-by: ClSlaid <cailue@bupt.edu.cn>

libs/clipboard/src/platform/linux/mod.rs

@@ -131,7 +131,6 @@ impl ClipboardContext {
 
         let mnt_opts = [
             MountOption::FSName("rustdesk-cliprdr-fs".to_string()),
-            MountOption::RO,
             MountOption::NoAtime,
         ];
         log::info!(