tr: doesn't warn about unescaped trailing backslash

[BenWiederhake]

$ true | tr '\\\' 'asdf'
tr: warning: an unescaped backslash at end of string is not portable
$ true | cargo run -q --features tr -- tr '\\\' 'asdf'
$

Found while reading #6713. Root cause is that the "escapedness"-property is being guessed based on the last two characters, even though it depends on whether the total amount of trailing backslashes is even or odd.

Ping @andrewliebenow.

Dibs, I wanna fix that myself. After writing all these reviews, I want to be a little bit "productive".