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 <email_idsname>. 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. developemnt and live 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>
Merge remote-tracking branch 'upstream/master' into emailheaders
made the email_idsname setting optional, no longer set by default

since this is optional feature install.sh no longer sets this by
default, if a user wants to use this feature they need to set it
themselves.

changed sendcustomemail.c and sendmail.c to only add the header to an
email if email_idsname is set to ensure a consistant behavior for
users upgrading.
Merge pull request #150 from dopefish/emailheaders

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