Allow specifying SPOOL_DIR path at runtime

[yakatz]

We would like to have two instances of cronie running, one for just the local machine and a separate one using an nfs mounted directory. A prerequisite for this would be the ability to change the SPOOL_DIR setting at runtime instead of only at compile time.

I understand this would get complicated for tools like crontab to edit the files - I would say at a minimum to leave that for the future and people who really want to do this can manage the files in the secondary spool directory some other way.

[t8m]

The simplest way would be to override the SPOOL_DIR via an environment variable. But that would work only for crond as crontab is setuid root.