Add optional "temp file transfer" module (defaults to "tempxfer"[.js])

This module (name configurable in SCFG->System->Loadable Modules) replaces the old hard-coded temp/archive file menu in v3.18 and earlier, going all the way back to v1a. Good-bye to yet more old and crufty Synchronet C source code. :-/

Change the "Pre Xtrn Prog" and "Post Xtrn Prop" menu option in SCFG->System->Loadable Modules to just "Pre Xtrn" and "Post Xtrn". The help text had the wrong names anyway (now fixed).
Save the menu lightbar position in the "Loadable Modules" SCFG menu.