* Em dash cleanup in pod/

* Em dash cleanup in pod/

I looked at all the instances of spaces around -- and in most cases
converted the sentences to use more appropriate punctuation. In
general, the -- in the perl docs seem to be there only to make
really complicated and really long sentences.

I didn't look at the closed em-dashes. They probably have the same
sentence-complexity problem.

I left some open em-dashes in place. Those are the ones used in
lists.