to avoid hanging in cleanup().

oct(0) should return '0', not '00'

* timemodule.c: the mac has no unistd.h
* classobject.c: fixed several cases of return NULL that should be
  return -1 !!!

  coercion is now completely generic.
* ceval.c: for instances, don't coerce for + and *; * reverses
  arguments if left one is non-instance numeric and right one sequence.

  the type object.

Real changes:
(a) added section on special class methods.
(b) corrected typos spotted by Stoffel Erasmus.

by special syntax: you can now define your own numbers, sequences and
mappings.

posixpath.py: added undocumented expanndvars() (expands $VAR in string).