nsenter,unshare: share common code; fix a bug of not closing all fds

nsenter,unshare: share common code; fix a bug of not closing all fds

function                                             old     new   delta
xvfork_parent_waits_and_exits                          -      64     +64
exec_prog_or_SHELL                                     -      39     +39
unshare_main                                         873     810     -63
nsenter_main                                         663     596     -67
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(add/remove: 2/0 grow/shrink: 0/2 up/down: 106/-130)          Total: -27 bytes

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