hush: fix nested redirects colliding with script fds

hush: fix nested redirects colliding with script fds

This necessitates switch from libc FILE api to a simple
homegrown replacement.
The change which fixes the bug here is the deleting of

	restore_redirected_FILEs();

line. It was prematurely moving (restoring) script fd#3.
The fix is: we don't even _want_ to restore scrit fds,
we are perfectly fine with them being moved.
The only reason we tried to restore them is that FILE api
did not allow moving of FILE->fd.

function                                             old     new   delta
refill_HFILE_and_getc                                  -      93     +93
hfopen                                                 -      90     +90
hfclose                                                -      66     +66
pseudo_exec_argv                                     591     597      +6
hush_main                                           1089    1095      +6
builtin_source                                       209     214      +5
save_fd_on_redirect                                  197     200      +3
setup_redirects                                      320     321      +1
fgetc_interactive                                    235     236      +1
i_peek_and_eat_bkslash_nl                             99      97      -2
expand_vars_to_list                                 1103    1100      -3
restore_redirects                                     99      52     -47
fclose_and_forget                                     57       -     -57
remember_FILE                                         63       -     -63
------------------------------------------------------------------------------
(add/remove: 3/2 grow/shrink: 6/3 up/down: 271/-172)           Total: 99 bytes

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