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.
On 11/06/2014 04:19 PM, Alexandre Detiste wrote:
>> @@ -287,14 +300,11 @@ def generate_timer_unit(job, seq):
>>              if os.path.isfile(REBOOT_FILE): return
>>              if delay == 0: delay = 1
>>              schedule = ''
>> -            persistent = False
> 
> a `@reboot` job can't be persistent

Makes sense.