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