xkbcomp: Enable xkbcomp-style octal escape sequences Xorg xkbcomp only parses octal sequences with `\0`, while xkbcommon does not force the `0` prefix of the numeric part. However, we only parsed up to to 3 digits, which does not allow to parse e.g. `\0377` while `\377` parses fine. Fixed by parsing up to 4 octal digits, while checking the result fits into a byte.