Fix performance issue on adm_config_report.php

In systems with large numbers of config items in mantis_config_table, the
Configuration Report page can take a very long time to load.

This behavior is due to each of the 'Delete' buttons being printed with
its own form, each one having a security token. The performance
bottleneck is actually the serialize/unserialize calls executed while
storing/retrieving the token from the PHP session.

To avoid this problem, the print_button() and form_security_field()
functions have been modified to accept a security token as an optional
parameter. This allows the calling page to generate a single token,
which is shared by all buttons.

Furthermore, print_button() also allows the security token parameter to
be 'OFF', which prevents the function from displaying a security field.
This is useful for buttons not resulting in modifications (i.e. not
requiring CSRF protection).

Fixes #13680