Convert start_blktrace to run_program

Rework start_blktrace and use run_program to launch blktrace. Move the
argv-building into the function so that it's easier to work with and
clean it up a bit. Add a signal parameter to wait_program to optionally
kill the pid with a given signal before waiting for it.

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