linking fs code problems in trunk

Hi all,
   Currently the fs related subsystem is broken in trunk, because of the
known linking problems.

I am proposing the attached patch which:
    - modifies the configure.in script to define in autoconf.h file the
HAVE_FS_UFS, HAVE_FS_AUFS, HAVE_FS_DISKD and HAVE_FS_COSS if the modules
ufs, aufs, diskd and coss will be build.
    - Makes the DiskdInstance UfsInstance and AufsInstance objects which
defined in src/fs/diskd/StoreFSdiskd.cc src/fs/ufs/StoreFSufs.cc and
src/fs/aufs/StoreFSaufs.cc files, non static
    - Adds the FsReg.cc file which just makes a reference to
DiskdInstance UfsInstance and AufsInstance objects to force the compiler
  to include the related objs and libraries into squid binary.

Regards,
      Christos