fix some strings quoting and remove some execs

install/dist/lib/debian60.lib.php

install/lib/update.lib.php

server/cron_daily.php

server/plugins-available/bind_plugin.inc.php

server/plugins-available/cron_jailkit_plugin.inc.php

server/plugins-available/cron_plugin.inc.php

server/plugins-available/getmail_plugin.inc.php

server/plugins-available/maildrop_plugin.inc.php

server/plugins-available/pma_symlink_plugin.inc.php

server/plugins-available/shelluser_base_plugin.inc.php

server/plugins-available/shelluser_jailkit_plugin.inc.php

server/plugins-available/software_update_plugin.inc.php

server/plugins-available/webmail_symlink_plugin.inc.php