db_script_exec: use a saved script name when reporting commands executed

Before executing a command in a ddb script ddb prints an information
line of the form:

db:1:my-script> command

where 1 is the script's depth level, "my-script" is the scipt's name,
and "command" is the current command in the script.
db_script_exec() uses its 'scriptname' parameter to produce that string.
In the case when db_script_exec() is called from db_run_cmd() the
argument points to db_tok_string that is a global variable used for
command parsing. So, its value changes with every command executed.

This commit changes the code to use the script's name stored in
ds_scriptname to print the line.

MFC after: 2 weeks