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.