- Add more documentation to mtx-changer.in

- Add more documentation to mtx-changer.in
- Correct link to manual in authenticate.c in various
  directories.
- Create a new src/stored/reserve.c file where the
  Use Storage command is processed and drives are
  reserved.
- Modify src/stored/autochanger.c to keep track of each
  Slot that is loaded for each device.
- Ensure that changer_command and changer_name are picked
  up from Autochanger resource if not specified, and if
  neither is specified, err.

git-svn-id: https://bacula.svn.sourceforge.net/svnroot/bacula/trunk@2100 91ce42f0-d328-0410-95d8-f526ca767f89