Fix/enhance PERSISTENT

Allow PERSISTENT envvar to take yes/no/auto with the following meaning:

'yes': force all further jobs to be persistent
'auto': only recognize @ keywords to be persistent
'no': force all further jobs not to be persistent

Defaults to 'yes' in anacrontab, and 'auto' for other sources.
this should stay in too things like 
`09,39 *     * * *     root   [ -x /usr/lib/php5/sessionclean ] && /usr/lib/php5/sessionclean`
never need to be persistent