Open stdin using its file descriptor so that a `dd skip=N` command in
a subshell does not consume all bytes from stdin.

For example, before this commit, multiple instances of `dd` reading
from stdin and appearing in a single command line would incorrectly
result in an empty stdin for each instance of `dd` after the first:

    $ printf "abcdef\n" | (dd bs=1 skip=3 count=0 && dd)  2> /dev/null
    # incorrectly results in no output

After this commit, the `dd skip=3` process reads three bytes from the
file descriptor referring to stdin without draining the remaining
three bytes when it terminates:

    $ printf "abcdef\n" | (dd bs=1 skip=3 count=0 && dd)  2> /dev/null
    def