Make set_directive and get_directive more strict
    
    When set_directive was used for directive "foo" and the word "foo"
    was detected anywhere on the line (e.g. in a comment, or in an example),
    it was overwritten which may potentially lead to wrong line being
    overwritten.
    
    Only match the directives on the beginning of the lines, it is safer.
    
    https://fedorahosted.org/freeipa/ticket/3974