* string.py: added rindex(), rfind(); changed index() to interpret

* string.py: added rindex(), rfind(); changed index() to interpret
  negative start indices starting from the right.
* ftplib.py: debug() -> set_debuglevel(); change demo to use __init__().
* os.py: added execl, execlp, and execvp.
* lambda.py: removed (now that we have built-in map, reduce, bagof, lambda)
* test_b{1,2}.py, testall.out: added tests for bagof, lambda, map, reduce
* commands.py: use os, not posix
* test_grammar.py: make it easy to disable non-portable int overflow tests
* dis.py: don't abuse range()