hush: add support for set -x

hush: add support for set -x

function                                             old     new   delta
run_pipe                                            1442    1568    +126
dump_cmd_in_x_mode                                     -     126    +126
builtin_trap                                         441     462     +21
pseudo_exec_argv                                     171     187     +16
reset_traps_to_defaults                              214     229     +15
check_and_run_traps                                  227     232      +5
hush_exit                                             98     101      +3
------------------------------------------------------------------------------
(add/remove: 1/0 grow/shrink: 6/0 up/down: 312/0)             Total: 312 bytes

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