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.