Add management of inifiles to allow manipulation of systemd units
    
    inifile_replace_variables() works similar to config_replace_variables() but
    allows to apply changes to specific section of an inifile. Inifiles are
    commonly used by freedesktop.org software and particularly used by systemd.
    
    When modifying inifile, all changes will be applied to specific section.
    
    Also fixes corner case in config_replace_variables() which would dublicate
    variables when adding them.