dd: fix a signal handling race

dd: fix a signal handling race

* src/dd.c (main): Install the signal handlers at startup
rather than just before the copy starts. In this way signals
received before the copy (like during a slow truncate for e.g.)
will be deferred and handled consistently.
* THANKS: Add Bernhard's email address.
* NEWS: Mention the fix.
Reported by Bernhard Voelker.