tr: [:upper:] and [:lower:] in SET1

[Per Starbäck]

tr (GNU coreutils) 6.9

  $ echo Foo | tr '[:upper:]' '[X*]'
  tr: misaligned [:upper:] and/or [:lower:] construct

Other character sets work like this, at least this one:

  $ echo Foo | tr '[:alnum:]' '[X*]'
  XXX

I guess there is a bug in code that is there to check if [:upper:] or
[:lower:] in SET2 is valid.

By the way,

  $ tr x '[:alnum:]'
  tr: when translating, the only character classes that may appear in
  string2 are `upper' and `lower'
  ^^^^^^^

I suggest "string2" -> "SET2" since that's what it's called in the
usage line.