hush: support "cd -- DIR" and such

hush: support "cd -- DIR" and such

function                                             old     new   delta
skip_dash_dash                                         -      33     +33
builtin_exit                                          43      48      +5
builtin_umask                                        121     125      +4
builtin_shift                                        115     119      +4
builtin_cd                                            71      75      +4
builtin_wait                                         271     274      +3
builtin_source                                       171     174      +3
builtin_exec                                          57      60      +3
builtin_eval                                          46      45      -1
------------------------------------------------------------------------------
(add/remove: 1/0 grow/shrink: 7/1 up/down: 59/-1)              Total: 58 bytes

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