hush: a few relatively trivial simplifications

hush: a few relatively trivial simplifications

function                                             old     new   delta
helper_export_local                                  130     135      +5
set_vars_and_save_old                                 89      85      -4
expand_variables                                     147     141      -6
get_ptr_to_local_var                                  77      70      -7
get_local_var_value                                  171     164      -7
delete_finished_bg_job                                31      16     -15
hush_exit                                            101      84     -17
free_pipe_list                                        31      12     -19
check_and_run_traps                                  232     205     -27
free_pipe                                            205     130     -75
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 1/9 up/down: 5/-177)           Total: -172 bytes

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