Add a new option to ktrdump (-l) which causes it to loop awaiting new data

Add a new option to ktrdump (-l) which causes it to loop awaiting new data
and then display it as it comes in.  Cleanup ktrdump considerably.  By
Aggelos Economopoulos.

Additional work by Matt:  Clean it up even more, document the code, and make
the new looping feature work with -s.

Submitted-by: Aggelos Economopoulos <aoiko@cc.ece.ntua.gr>
With-additional-work-by: dillon