Use libfmt to display format strings coming from server >= v18

[kambala-decapitator]

Continuation of #487

[kambala-decapitator]

I think this would be good for iOS < 11 users, can go into next release. @wutschel what do you think?

[wutschel]

Well, I am ok adding this for the next release. Even though it still feels like cracking a nut with a sledgehammer -- it is only about adding the unit of a setting ("%" or "ms") for some rarely used settings. Would you handle this?

[kambala-decapitator]

oh, I thought it's a more general case. Yeah, then can go in later.