Handle more mem alloc issues found with failmalloc

[The other half of this patch affected Python/symtable.c, and wasn't
 relevant for the 2.4 branch. --amk]

"from blah import (foo, bar
baz, bongo)"

Check return value from future_parse() in for loop for file_input to
accomodate multiple future statements on separate lines.

Add several comments explaining how the code works.

Remove out-dated XXX comment.

Replace uses of PyCF_xxx with CO_xxx.

Replace individual feature slots in PyFutureFeatures with single
bitmask ff_features.

When flags must be transfered among the three parts of the interpreter
that care about them -- the pythonrun layer, the compiler, and the
future feature parser -- can simply or (|) the definitions.

This introduces:

- A new operator // that means floor division (the kind of division
  where 1/2 is 0).

- The "future division" statement ("from __future__ import division)
  which changes the meaning of the / operator to implement "true
  division" (where 1/2 is 0.5).

- New overloadable operators __truediv__ and __floordiv__.

- New slots in the PyNumberMethods struct for true and floor division,
  new abstract APIs for them, new opcodes, and so on.

I emphasize that without the future division statement, the semantics
of / will remain unchanged until Python 3.0.

Not yet implemented are warnings (default off) when / is used with int
or long arguments.

This has been on display since 7/31 as SF patch #443474.

Flames to /dev/null.

pythonrun.c, run_pyc_file():  repair semantic error wrt CO_GENERATOR vs
CO_GENERATOR_ALLOWED.

that info to code dynamically compiled *by* code compiled with generators
enabled.  Doesn't yet work because there's still no way to tell the parser
that "yield" is OK (unlike nested_scopes, the parser has its fingers in
this too).
Replaced PyEval_GetNestedScopes by a more-general
PyEval_MergeCompilerFlags.  Perhaps I should not have?  I doubted it was
*intended* to be part of the public API, so just did.

the yield statement.  I figure we have to have this in before I can
release 2.2a1 on Wednesday.

Note: test_generators is currently broken, I'm counting on Tim to fix
this.