all flag variables initialized with zero
respond `Y' is equal to `y'
update usage string
prompt only if source exist
ignore -i option if file descriptor stdin not refers to a valid
terminal type device
`mv foo/ ../..' to `mv foo/ ../../foo/', not to `mv foo/ ../../'. The
latter caused a panic. Before the trailing slash changes in the kernel,
the trailing slashes caused the rename() for this mv to fail earlier, so
there was no panic in 2.0.
Fixes part of PR 760.
