hush: move variable expansion into a separate function. No logic changes

hush: move variable expansion into a separate function. No logic changes

function                                             old     new   delta
expand_one_var                                         -    1551   +1551
expand_vars_to_list                                 2833    1175   -1658
------------------------------------------------------------------------------
(add/remove: 1/0 grow/shrink: 0/1 up/down: 1551/-1658)       Total: -107 bytes

Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>