loadTestsFromName to accept new-style classes too!

I seriously doubt this will break any deployed code.

Docstring consistency with the updated .tex files.

semantics of header chunks using byte and Unicode strings.
Specifically,

append(): When the given string is a byte string, charset (whether
specified explicitly in the argument list or implicitly via the
constructor default) is the encoding of the byte string, and a
UnicodeError will be raised if the string cannot be decoded with that
charset.  If s is a Unicode string, then charset is a hint specifying
the character set of the characters in the string.  In this case, when
producing an RFC 2822 compliant header using RFC 2047 rules, the
Unicode string will be encoded using the following charsets in order:
us-ascii, the charset hint, utf-8.

__init__(): Use the global USASCII Charset instance when the charset
argument is None.  Also, clarification in the docstring.

Also, use True/False where appropriate.

Relax regular expression to handle spaces in user and group names.

    ``void (*)(PyObject *)``

isn't quite the same as

    void (*)(PyObject \*)

but I gather ``...`` is preferable stylistically in most cases than
... \*...

Python 2.1.3.  However it's required by the email tests suite, so poke
it into the encodings aliases if it's missing.  The is apparently the
approved API for doing so.

Now we can remove the hexversion shortcircuits in the test suite.

Use -b HEAD to limit output to the trunk (skip all branch revisions).

doesn't know about the ansi-x3.4-1968 charset so skip two tests that
rely on that (msg_32.txt and msg_33.txt).

some of the test values which change because of this.

filename of the current buffer.

encoding flag SHORTEST means to return the shortest encoding between
base64 and qp.  This is used for the header_enc for utf-8.  SHORTEST
isn't legal for body_enc.

Also some code cleanup:

- use True/False everywhere
- use == instead of `is' in a few places
- added _unicode() and make consistent the "is unicode" checks
- update docstrings

Closes SF bug #583248; backporting to r22-maint branch.