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.
diff --git a/options.h b/options.h
index 6c28ed9..d0df3a7 100644
--- a/options.h
+++ b/options.h
@@ -1,6 +1,6 @@
 /*
  * This file is part of ltrace.
- * Copyright (C) 2012 Petr Machata, Red Hat Inc.
+ * Copyright (C) 2012,2013 Petr Machata, Red Hat Inc.
  * Copyright (C) 2009,2010 Joe Damato
  * Copyright (C) 1998,2002,2008 Juan Cespedes
  * Copyright (C) 2006 Ian Wienand
@@ -103,12 +103,6 @@
 /* Vector of struct opt_F_t.  */
 extern struct vect opt_F;
 
-struct opt_c_struct {
-	int count;
-	struct timeval tv;
-};
-extern struct dict *dict_opt_c;
-
 extern char **process_options(int argc, char **argv);
 
 #endif /* _OPTIONS_H_ */