Add configuration file permission validation and access-log parameter

Since this daemon probably will be run as root some day, it's
necessary to ensure that the config file cannot be altered by
other users than the one running the script - thus protecting
against other users being able to fool the script into executing
arbitrary commands.

access-log parameter added as Net::HTTPServer by default writes
to ./access.log - this is a way to change that.
Merge branch 'config-validation' of /home/git/repositories/diddi/gitlab-autopull