Supports locale-specified encoding for rcfile.

This commit fixes issue #1317.  The locale for the encoding is obtained from
the Python `locale` module, which can be specified by the `LANG` environment
variable.

This was tested by adding a test rc file encoded in UTF-32-BE, and testing that
it decoded correctly.
Ah, I misunderstood your question. This test works fine on my mac which doesn't support this locale. Which is why I think our earlier approach of `locale.getpreferredencoding()` failed.