meson: allow to customize the access mode for tty/pts devices

Then, switch the default value to "0600", due to general security
concerns about terminals being written to by other users.

Closing #35599.

src/nspawn/nspawn.c

@@ -2399,13 +2399,13 @@ static int setup_pts(const char *dest) {
 #if HAVE_SELINUX
         if (arg_selinux_apifs_context)
                 (void) asprintf(&options,
-                                "newinstance,ptmxmode=0666,mode=620,gid=" GID_FMT ",context=\"%s\"",
+                                "newinstance,ptmxmode=0666,mode=" STRINGIFY(TTY_MODE) ",gid=" GID_FMT ",context=\"%s\"",
                                 arg_uid_shift + TTY_GID,
                                 arg_selinux_apifs_context);
         else
 #endif
                 (void) asprintf(&options,
-                                "newinstance,ptmxmode=0666,mode=620,gid=" GID_FMT,
+                                "newinstance,ptmxmode=0666,mode=" STRINGIFY(TTY_MODE) ",gid=" GID_FMT,
                                 arg_uid_shift + TTY_GID);
 
         if (!options)