Unify how file mode is specified

The lines of code I commented on in #10254 were meant to serve only as examples
of the changes I was requesting, not the only instances.

Also just use `Mode::from_bits_truncate()` instead of unsafe or unwrapping since
we know the modes are correct.

src/autoload.rs

@@ -341,7 +341,7 @@ fn test_autoload() {
         let fd = wopen_cloexec(
             path,
             OFlag::O_RDWR | OFlag::O_CREAT,
-            Mode::from_bits(0o666).unwrap(),
+            Mode::from_bits_truncate(0o666),
         )
         .unwrap();
         write_loop(&fd, "Hello".as_bytes()).unwrap();

src/env_universal_common.rs

@@ -441,11 +441,7 @@ impl EnvUniversal {
 
         let mut res_fd = None;
         while res_fd.is_none() {
-            let fd = match wopen_cloexec(
-                &self.vars_path,
-                flags,
-                Mode::S_IRUSR | Mode::S_IWUSR | Mode::S_IRGRP | Mode::S_IROTH,
-            ) {
+            let fd = match wopen_cloexec(&self.vars_path, flags, Mode::from_bits_truncate(0o644)) {
                 Ok(fd) => fd,
                 Err(err) => {
                     if err == nix::Error::EINTR {

src/history.rs

@@ -128,7 +128,7 @@ const HISTORY_SAVE_MAX: NonZeroUsize = unsafe { NonZeroUsize::new_unchecked(1024
 const HISTORY_OUTPUT_BUFFER_SIZE: usize = 64 * 1024;
 
 /// The file access mode we use for creating history files
-const HISTORY_FILE_MODE: Mode = Mode::S_IRUSR.union(Mode::S_IWUSR);
+const HISTORY_FILE_MODE: Mode = Mode::from_bits_truncate(0o600);
 
 /// How many times we retry to save
 /// Saving may fail if the file is modified in between our opening

src/io.rs

@@ -1007,7 +1007,7 @@ const FILE_ERROR: &wstr = L!("An error occurred while redirecting file '%ls'");
 const NOCLOB_ERROR: &wstr = L!("The file '%ls' already exists");
 
 /// Base open mode to pass to calls to open.
-const OPEN_MASK: Mode = unsafe { Mode::from_bits_unchecked(0o666) };
+const OPEN_MASK: Mode = Mode::from_bits_truncate(0o666);
 
 /// Provide the fd monitor used for background fillthread operations.
 fn fd_monitor() -> &'static mut FdMonitor {