A function that turns a Swift 3 [String] to an array of C strings (char *[]).

withCStrings.swift

func withCStrings(_ strings: [String], scoped: ([UnsafeMutablePointer<CChar>?]) throws -> Void) rethrows {
    let cStrings = strings.map { strdup($0) }
    try scoped(cStrings + [nil])
    cStrings.forEach { free($0) }
}