tr(1): backslash(): fix octal escape parsing

There are two bugs in backslash():

1. 8 and 9 are not octal digits.  If we see '8' or '9' we should
   terminate the octal escape.

2. We return octal escape values larger than UCHAR_MAX even though
   tr(1) is (currently) a byte-oriented program and values larger
   than UCHAR_MAX make no sense.

So, fix them both.  In particular, (a) stop parsing if we see
characters outside of '0'-'7' and (b) escaped octal values larger than
UCHAR_MAX are a terminal error.

While here, some cleanup:

- Check for empty escapes at the top of the function.  This simplifies
  later cases.

- Use the for-loop conditional to terminate octal escape parsing after
  three characters.

- Use an ANSI-style function definition.

We can fix the switch-statement indentation later, in a larger KNF
patch.

ok millert@

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