Move CD-ROM handling backend into libapt-pkg

Move CD-ROM handling backend into libapt-pkg
    - apt-cdrom.cc seperated into frontend (cmdline/apt-cdrom.cc and library
      apt-pkg/cdrom.{cc,h}) (Ubuntu #5668)

Patches applied:

 * michael.vogt@ubuntu.com--2005/apt--auth-cdrom--0--base-0
   tag of apt@packages.debian.org/apt--main--0--patch-51

 * michael.vogt@ubuntu.com--2005/apt--auth-cdrom--0--patch-1
   * added support for signed cdroms

 * michael.vogt@ubuntu.com--2005/apt--auth-cdrom--0--patch-2
   * merged with apt--main, seperated cmdline/apt-cdrom.cc into a library (apt-pkg/cdrom.{cc,h})

 * michael.vogt@ubuntu.com--2005/apt--auth-cdrom--0--patch-3
   * cleaned up the cmdline/apt-cdrom.cc code