bug#27786: tr: accept hex ranges as well as octal

[積丹尼 Dan Jacobson]

In (info "(coreutils) Character sets")

     ‘\OOO’
          The 8-bit character with the value given by OOO, which is 1 to
          3 octal digits.  Note that ‘\400’ is interpreted as the
          two-byte sequence, ‘\040’ ‘0’.

OK but do add hex too. Hmm, I probably reported this before but can't
tell because I'm offline while I'm writing this, and it is too important
anyway for me to check vs. mentioning again.