hush: implement brace expansion

hush: implement brace expansion

When enabled:

function                                             old     new   delta
glob_brace                                             -     402    +402
next_brace_sub                                         -      70     +70
expand_on_ifs                                        185     231     +46
bbconfig_config_bz2                                 4923    4929      +6
o_save_ptr                                           282     140    -142
------------------------------------------------------------------------------
(add/remove: 2/0 grow/shrink: 2/1 up/down: 524/-142)          Total: 382 bytes

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