ash: significant overhaul of redirect saving logic

ash: significant overhaul of redirect saving logic

New code is similar to what hush is doing.
Make CLOSED to -1: same as dash.
popredir() loses "restore" parameter: same as dash.
COPYFD_RESTORE bit is no longer necessary.

This change fixes this interactive bug:

	$ ls -l /proc/$$/fd 10>&-
	ash: can't set tty process group: Bad file descriptor
	ash: can't set tty process group: Bad file descriptor
	[1]+  Done(2)                    ls -l /proc/${\$}/fd 10>&4294967295

function                                             old     new   delta
unwindredir                                           29      27      -2
tryexec                                              154     152      -2
evaltree                                             503     501      -2
evalcommand                                         1369    1367      -2
cmdloop                                              187     185      -2
redirect                                            1029    1018     -11
popredir                                             153     123     -30
need_to_remember                                      36       -     -36
is_hidden_fd                                          68       -     -68
------------------------------------------------------------------------------
(add/remove: 0/2 grow/shrink: 0/7 up/down: 0/-155)           Total: -155 bytes
   text    data     bss     dec     hex filename
 914572     485    6848  921905   e1131 busybox_old
 914553     485    6848  921886   e111e busybox_unstripped

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