hush: convert exp/ro/local parameters to bitfields in one flag param

hush: convert exp/ro/local parameters to bitfields in one flag param

function                                             old     new   delta
helper_export_local                                  174     185     +11
set_local_var                                        424     420      -4
run_list                                            1048    1044      -4
set_vars_and_save_old                                 88      83      -5
set_local_var_from_halves                             27      22      -5
run_pipe                                            1554    1549      -5
builtin_export                                       173     168      -5
set_pwd_var                                           40      34      -6
builtin_readonly                                      70      64      -6
expand_one_var                                      1625    1618      -7
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 1/9 up/down: 11/-47)            Total: -36 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>