hush: implement "readonly" builtin

hush: implement "readonly" builtin

function                                             old     new   delta
builtin_readonly                                       -      70     +70
helper_export_local                                  152     174     +22
bltins1                                              348     360     +12
expand_one_var                                      1620    1625      +5
builtin_export                                       168     173      +5
set_pwd_var                                           36      40      +4
set_local_var                                        410     414      +4
set_vars_and_save_old                                 85      88      +3
set_local_var_from_halves                             24      27      +3
run_pipe                                            1551    1554      +3
run_list                                            1046    1048      +2
builtin_type                                         116     114      -2
------------------------------------------------------------------------------
(add/remove: 1/0 grow/shrink: 10/1 up/down: 133/-2)           Total: 131 bytes

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