Use secure_path in sudoers to fix a local privilege escalation bug

Our system mail handler runs as root using sudo, however the code
responsible for getting the software name (/etc/rc.freenas) runs
commands using relative path, which is vulnerable to PATH changes.

This change makes sure PATH environment var is reset to a sane default
prior to /etc/find_alias_for_smtplib.py execution.

Reported by:	William Söderberg <william.soderberg cuebid se>