target: use correct target in target-prefixed commands and event handlers

This change contains an alternative to Matthias Welwarsky's #4130
(target-prefixed commands) and to #4293 (event handlers).

get_current_target() must retrieve the target associated to the current
command. If no target associated, the current target of the command
context is used as a fallback.

Many Tcl event handlers work with the current target as if it were
the target issuing the event.

current_target in command_context is a number and has to be converted
to a pointer in every get_current_target() call.

The solution:
- Replace current_target in command_context by a target pointer
- Add another target pointer current_target_override
- get_current_target() returns current_target_override if set, otherwise
	current_target
- Save, set and restore current_target_override to the current prefix
	in run_command()
- Save, set and restore current_target_override to the event invoking
	target in target_handle_event()

While on it use calloc when allocating a new command_context.

Change-Id: I9a82102e94dcac063743834a1d28da861b2e74ea
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Suggested-by: Matthias Welwarsky <matthias.welwarsky@sysgo.com>
Reviewed-on: http://openocd.zylin.com/4295
Tested-by: jenkins
Reviewed-by: Matthias Welwarsky <matthias@welwarsky.de>

src/server/tcl_server.c

@@ -157,7 +157,7 @@ static int tcl_new_connection(struct connection *connection)
 
 	connection->priv = tclc;
 
-	struct target *target = get_target_by_num(connection->cmd_ctx->current_target);
+	struct target *target = get_current_target(connection->cmd_ctx);
 	if (target != NULL)
 		tclc->tc_laststate = target->state;