DB upgrade can break user script path

[bouthilx]

When upgrading the DB, the parser data will be updated and absolute path of the main script will be inferred based on the commandline call of the experiment. This can have the effect of changing the absolute path to a different one if the file is present but the upgrade is done on another file system than the experiment. The update of the parser should not lead to an absolute path relative to the system in use for the upgrade, the path should either be left as is or reverted to relative path given in command line.

This issue was the cause of #553 .

@mgermain

[bouthilx]

Related, an absent user config file will lead into flushing the search space because of this part. The DB upgrade should not depend on any local files.