Andrew quite correctly notices that the next() method isn't quite what
we need, since it returns None upon end instead of raising
StopIteration.  His fix is easy enough, using iter(self.next, None)
instead.

appropriate next() method, and this is what people really want to do with
these objects in practice.

cut-and-paste copy of the seek() method on the _Subfile class, but it
didn't make one bit of sense: it sets self.pos, which is not used in
this class or its subclasses, and it uses self.start and self.stop,
which aren't defined on this class or its subclasses.  This is purely
my own fault -- I added this in rev 1.4 and apparently never tried to
use it.  Since it's not documented, and of very questionable use given
that there's no tell(), I'm ripping it out.

This resolves SF bug 416199 by Andrew Dalke: mailbox.py seek problems.

- All constructors grow an optional argument `factory' which is a
  callable used when new message instances are created by the next()
  methods.  Defaults to the rfc822.Message class.

- A new subclass of UnixMailbox is added, called PortableUnixMailbox.
  It's identical to UnixMailbox, but uses a more portable test for
  From_ delimiter lines.  With PortableUnixMailbox, any line that
  starts with "From " is considered a delimiter (this should really
  check for two newlines before the F, but it doesn't.

Use != instead of <> since <> is documented as "obsolescent".
Use "is" and "is not" when comparing with None or type objects.

This closes SourceForge bug #117490.

This closes SourceForge patch #101714.

                     class conforms to the maildir specification.

mailbox.py (from the CVS tree) doesn't work with qmail Maildirs:
Filenames are completed when the directories are scanned, and
the directory name is prepended again in the next() method.

Another suggestion: Change the print statement in the _test()
driver to show two more date characters (probably the length
has increased due to the recent Y2K hype ;). Now it shows the
complete date, including the seconds -- at least for me. (I've
also made the sender field left justified, in case it is ever
shorter than the field width).

Also don't allow leading zeros in message numbers.

mailbox.py: Convert to 4-space indents.

Unix From lines, change the UnixMailbox class so that _search_start()
positions the file *before* the Unix From line instead of after it;
change _search_end() to skip one line before looking for the next From
line.  The rfc822.Message class automatically recognizes these Unix
From lines and squirrels them away in the 'unixfrom' instance variable.

are, we must keep the file around so we can print the body.

Chris Lawrence sent me a broken version; this one is a tad simpler and
more conforming to the standard.

Add a class to mailbox.py for dealing with qmail directory mailboxes.
The test code was extended to notice these being used as well.

fp.tell() -- that won't work on Windows.

(A patch for rfc822 is still needed for one case where it finds a bad
header line and wants to back up.)

don't put the seconds in the time!

allow negative numbers to specify read until EOF (like for a regular
file's read() method).

otherwise don't look like headers at all...
Also robustify the test code a bit.

Added optional third parameter to seek.