hush: partially fix wrong expansion on $IFS (bug 4027).

hush: partially fix wrong expansion on $IFS (bug 4027).

In the added testcase, before patch we failed 8 out of 9 tests,
now we fail only 2 (4th and 5th).

function                                             old     new   delta
expand_on_ifs                                        225     258     +33
expand_vars_to_list                                 1038    1054     +16
o_save_ptr_helper                                    115     119      +4
builtin_umask                                        132     133      +1
o_addQstr                                            165     161      -4
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(add/remove: 0/0 grow/shrink: 4/1 up/down: 54/-4)              Total: 50 bytes

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