Turn =item commands into =begin/=end item by default.

May not be perfect, seeing as termination occurs only when any of =over,
=item, or =back are being processed. (e.g. a =head1 in an item list
won't end a =begin item, and will prevent it from being ended)
Make X<> work, other random fixes

Turns POD X<foo> into X<|foo>. Also some fixes involving =item being now
a =begin/=end item.
Never print "back" as a command

Mirrors the "over" fix in the last commit :) .