Merge branch 'stable-3.1' of https://git.ispconfig.org/ispconfig/ispconfig3 into stable-3.1

install/dist/lib/fedora.lib.php

install/dist/lib/opensuse.lib.php

install/lib/installer_base.lib.php

install/tpl/apache_ispconfig.vhost.master

install/tpl/debian6_dovecot2.conf.master

install/tpl/debian_dovecot2.conf.master

install/tpl/debian_postfix.conf.master

install/tpl/fedora_dovecot2.conf.master

install/tpl/fedora_postfix.conf.master

install/tpl/gentoo_postfix.conf.master

install/tpl/mysql-virtual_alias_domains.cf.master

install/tpl/mysql-virtual_alias_maps.cf.master

install/tpl/nginx_apps.vhost.master

install/tpl/nginx_ispconfig.vhost.master

install/tpl/opensuse_dovecot2.conf.master

install/tpl/opensuse_postfix.conf.master

server/plugins-available/apache2_plugin.inc.php

server/plugins-available/nginx_plugin.inc.php