BibSched/BibTasks to run only with Apache user credentials.

BibSched/BibTasks will now run only with user credentials compatible with
the user running the Apache webserver processes. This is necessary to
avoid incompatible file creation/modification on the filesystems.
In case the user want to force a particular username she can use the variable
CFG_BIBSCHED_PROCESS_USER in invenio(-local).conf.