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.
Merge pull request #37 from wavexx/tristate_persistent

Fix/enhance PERSISTENT