install: -t without -D writes the file as the target when the target is not a directory

[abendrothj]

Without -D, install -t X f should fail unless X is an existing directory. GNU does: install: failed to access 'X': Not a directory (or No such file or directory). With a single source, uutils instead installs f as X and exits 0:

echo old > regular; ln -s nowhere dangling; mkfifo fifo; echo new > f
install -t regular f     # regular now contains "new"
install -t dangling f    # the symlink is replaced by a regular file
install -t fifo f        # the fifo is replaced by a regular file
install -t missing f     # creates a file named "missing"

The same happens with --target-directory=X, and with -v (which prints removed 'regular'), -b (which backs up and then overwrites) and -C. With two or more sources the target is rejected as expected (target 'regular' is not a directory).

GNU fails in all of these cases and leaves the paths untouched. A mistyped -t silently overwrites a file, similar to #12926.