Overhaul the help output and display valid lib names when helpful

- If sysop specifies an invalid lib name, show them what the valid names are
- Make it clear that the directory specified (dir-spec) is either a combination
  of -lib and -dir *or* -all *or* dir-code *or* nothing (a get prompted)
- When using -all, clear the list of specified directories to prevent dupe
  dir scanning when misused.