Add optional support for the CCC (Clear Command Channel) command.

Add optional support for the CCC (Clear Command Channel) command.
This command is dangerous, since it allows commands to be transmitted
without integrity checking, so the default behavior without the -c
option is still for the server to refuse to accept unprotected
commands.

    * ftpd.c: Add a new command line option, -c, which tells the
    server to accept the CCC command.

    * ftpcmd.y: If the -c option was given, check to make sure the CCC
    command itself was integrity protected, and then set ccc_ok to
    allow future commands to be transmitted as cleartext.
    (getline): Now that CCC is potentially allowed, we must check to
    see if we are parsing an unprotected command even if a security
    context is established (i.e. auth_type is set).

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@10846 dc483132-0cff-0310-8789-dd5450dbe970