iowatcher: Merge trace dumping functions into one

iowatcher: Merge trace dumping functions into one

Now that combine_blktrace_devs() takes a list of traces it's fairly
generic so we might as well merge blktrace_to_dump() into it. The latter
can be replaced with a call using a list with a single entry.

combine_blktrace_devs() is renamed dump_traces() because that's what it
does.

Also eradicate the big global char array 'line' that was being used in a
bunch of places along with some more unnecessary strdup()s.

Signed-off-by: Andrew Price <anprice@redhat.com>