hush: fix/explain corner cases of redirection colliding with script fd

hush: fix/explain corner cases of redirection colliding with script fd

function                                             old     new   delta
save_fd_on_redirect                                  200     208      +8
run_pipe                                            1870    1873      +3
setup_redirects                                      321     322      +1
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(add/remove: 0/0 grow/shrink: 3/0 up/down: 12/0)               Total: 12 bytes

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