hush: make "set -x" output don't redirectable when fd#2 redirected

hush: make "set -x" output don't redirectable when fd#2 redirected

function                                             old     new   delta
x_mode_print_optionally_squoted                        -     120    +120
x_mode_flush                                           -      68     +68
save_fd_on_redirect                                  208     243     +35
x_mode_prefix                                          -      27     +27
x_mode_addblock                                        -      23     +23
x_mode_addchr                                          -      17     +17
dump_cmd_in_x_mode                                   110      85     -25
run_pipe                                            1919    1890     -29
print_optionally_squoted                             145       -    -145
------------------------------------------------------------------------------
(add/remove: 5/1 grow/shrink: 1/2 up/down: 290/-199)           Total: 91 bytes

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