[IMP] tools: support known "falsy" string values for XML boolean fields

Historically, developers had to use the `eval` attribute to set
a False value for boolean fields in XML data files:

    <field name="active" eval="False"/>

This is a source of errors for beginners, who tend to provide
a string value directly, as for char/text fields.
Unfortunately string values always evaluated to `True` booleans:

    <field name="active">False</field> <!-- active == true! -->

This patch adds detection of some well-known falsy strings
(case-insensitive): `0`, `false`, `off`, as similarly supported
for the `noupdate` and `forcecreate` attributes of XML data records.

Closes #13152