* Adds a "clean" target to the Makefile

* In order to allow a module to be both a module and a client, all
  module-only functions are renamed to begin with "module_", and all
  client-only functions are renamed to begin with "client_".  These
  new functions are defined in "module2.h" and "client2.h".  The old
  names are #define'd to the new ones in "module.h" and "client.h",
  for compatiblity.

* A new module, cvm-anon, is created.  This module does anonymous
  authentication.  Any users whose names appear in CVM_ANON_USERS are
  accepted without any password at all; if a username is followed by
  an equal sign and another username, any user logging in as the first
  username will be treated as if they had logged in anonymously to the
  second username.  If CVM_ANON_NEXT is set, the environment variable
  CVM_ALREADYAUTH is set, the username is passed onto the module whose
  name is there to retrieve the other information.  This module must
  recognize CVM_ALREADYAUTH as meaning that the user is already
  authenticated (cvm-unix has been modified in this way; other modules
  will need similar modifications to be used in this way), and should
  not have their password checked again.  If CVM_ANON_NEXT isn't set,
  the user's information is taken from the environment variables
  CVM_ANON_USER, CVM_ANON_SHELL, CVM_ANON_UID, and CVM_ANON_GID.

* A new module, cvm-chain, is created.  This module runs a sequence of
  CVM modules until one succeeds.  It gets the list of modules to try
  from the CVM_CHAIN environment variable, which should contain a
  space-seperated list of modules to try, in the order they should be
  tried.

* The cvm-unix module is modified to recognize the environment
  variable CVM_ALREADYAUTH as indicating that the user has already
  authenticated some other way, and should just have their settings
  retrieved.