Fixed HttpHdr::Private/NoCache(v) implementations and optimized their API.

Fixed HttpHdr::Private/NoCache(v) implementations and optimized their API.

These calls now avoid assertions and extra trailing commas when called with
empty names. The API now allows calling with a String() object, but still
needs more polishing work.

Moved common code from Ftp::Server::setReply and Ftp::Relay::createHttpReply()
into Ftp::HttpReplyWrapper().

Also removed the last non-job callbak from Ftp::Client, polished and synced
new FTP comments with the modern client/server/gateway/relay terminology, as
well as minimized changes compared to trunk.