Consider exec and exit events an end of outstanding calls

- This cleans up a lot of stuff.  The actual substance is addition of
  account_current_callstack in handle_event.c (which however uses
  those cleaned-up interfaces).

- trace-exec.exp was extended to check that the exec syscall can be
  seen in -c output.  That's one of the symptoms of what this fixes.

- This hides dict_opt_c in summary.c.  It also gets rid of the global
  variable current_time_spent--instead, the information is passed via
  a function argument in a struct timedelta (so that we never confuse
  absolute time with relative).  callstack_element.time_spent was renamed
  to .enter_time, as that is what it has always been.