hush: IFS fixes

hush: IFS fixes

$ IFS=": "; x=" "; set x $x; for v; do echo "|$v|"; done
|x|
$ IFS=": "; x=":"; set x $x; for v; do echo "|$v|"; done
|x|
||

function                                             old     new   delta
run_pipe                                            1789    1870     +81
expand_on_ifs                                        310     361     +51
pseudo_exec_argv                                     588     591      +3
builtin_local                                         50      53      +3
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 4/0 up/down: 138/0)             Total: 138 bytes

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