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.
Gotcha.  Since we're using `getdefaultlocale` rather than `getpreferredencoding`, we should be ok, and my concern is probably moot.