Added <email_idsname> option to ossec.conf (additional email header)

This feature adds an additional option to the ossec_config/global config
block in ossec.conf called &lt;email_idsname&gt;. The value of this field gets
added o the email headers as "X-IDS-OSSEC: $value" to make sorting of
emails from different masters easier (e.g. development and production masters).
install.sh uses the $HOST variable as the default value for the field when 
creating an ossec.conf

Example:

    <ossec_config>
      <global>
        <email_idsname>development</email_idsname>
      </global>
    </ossec_config>