Add the new global options '--password-file' and '--password-envvar'.

* subversion/svn/svn.c
  (svn_cl__longopt_t,svn_cl__global_options): Add opt_auth_password_envvar
    and opt_auth_password_file.
  (svn_cl__options): Add the '--password-file' and '--password-envvar' options,
    and add a warning to the '--password' option.
  (sub_main): Add opt_auth_password_envvar and opt_auth_password_file,
    also check for multiple use of stdin.