Installer: Gentoo updates

• Added configure_dovecot function
• Updated functions configure_postfix, configure_pureftpd, configure_apache, install_ispconfig, configure_apps_vhost
• Bug fix in configure_amavis where installer was not including the ispconfig config file
• Bug fix in configure_apache where in suphp.conf the php handler line would be duplicated through updates
• replace double-quoted strings with single-quoted whenever appropriate
• changed inline comments to coding standards
• fixed OS specific paths
• changes to installer_base 1) add_database_server_record added os specific override of awstats paths 2) find_installed_apps courier not detecting correctly for gentoo