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.
Ok, so this code should move in parse_crontab and be run only when PERSISTENT=auto
( 'M','w','d','h' have to be stored in 4 temporary values before the yeld)