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.