libutil/unescape: Stop octal escape at 3 digits

unescape() is used by several tools, in particular printf(1) and
tr(1), which should stop the octal escape at a maximum of 3 digits:

printf(1)
> In addition to the escape sequences shown in XBD 5. File Format
> Notation ('\\', '\a', '\b', '\f', '\n', '\r', '\t', '\v'), "\ddd",
> where ddd is a one, two, or three-digit octal number, shall be
> written as a byte with the numeric value specified by the octal
> number.

tr(1)
> An octal sequence shall consist of a <backslash> followed by the
> longest sequence of one, two, or three-octal-digit characters.

Previously, the maximum was set to 4 (possibly a typo?), which meant
that printf '\0123' printed `S` instead of `<newline>3`.

To check that this doesn't break any other tools using unescape:

- cut: used for -d parameter, escapes are non-standard
- join: used for -t parameter, escapes are non-standard
- nl: used for -s parameter, escapes are non-standard
- paste: used for -d parameter, POSIX specifies \n, \t, \\, and \0,
  \0 followed by a digit is unspecified
- sort: used for -t parameter, escapes are non-standard

libutil/unescape.c

@@ -37,7 +37,7 @@ unescape(char *s)
 		} else if (escapes[(unsigned char)*r]) {
 			*w++ = escapes[(unsigned char)*r++];
 		} else if (is_odigit(*r)) {
-			for (q = 0, m = 4; m && is_odigit(*r); m--, r++)
+			for (q = 0, m = 3; m && is_odigit(*r); m--, r++)
 				q = q * 8 + (*r - '0');
 			*w++ = MIN(q, 255);
 		} else if (*r == 'x' && isxdigit(r[1])) {