Keep legit code working even when --screw-ie is not passed.

Previously:

    Without `--screw-ie`, UglifyJS would always leak names of function
    expressions into the containing scope, as if they were function
    declarations.  That was to emulate IE<9 behavior.  Code relying on this
    IE bug would continue to work properly after mangling, although it would
    only work in IE (since other engines don't share the bug).  Sometimes
    this broke legitimage code (see #153 and #155).

    With `--screw-ie` the names would not be leaked into the current scope,
    working properly in legit cases; but still it broke legit code when
    running in IE<9 (see #24).

Currently:

    Regardless of the `--screw-ie` setting, the names will not be leaked.
    Code relying on the IE bug will not work properly after mangling.
    <evil laughter here>

    Without `--screw-ie`: a hack has been added to the mangler to avoid
    using the same name for a function expression and some other variable in
    the same scope.  This keeps legit code working, at the (negligible,
    indeed) cost of one more identifier.

    With `--screw-ie` you allow the mangler to name function expressions
    with the same identifier as another variable in scope.  After mangling
    code might break in IE<9.

Oh man, the commit message is longer than the patch.

Fix #153, #155