feature request to support stdin in GNU install

[Dröge , Lars]

Dear coreutils developers,

I have read many tutorials, where a secret is written to a file, which
is protected afterwards, like this:

```
generate_secret > keyfile
chmod 0600 keyfile
```

I hope that the following approach would have been intuitive and more
secure, as the keyfile was never readable:

```
generate_secret | install -x -m 0600 keyfile
```

where "-x" would indicate, that stdin shall be used as source. Please
take "x" as a placeholder; any letter works for me.

Do you like the idea to support stdin?

Best regards, 
Lars

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