src/pre: Use sed(1) and handle buffering with stdbuf(1)

The while loop and its variables where trimming the string undesirably:

    $ printf '\ta\n b\n' | pre -n out
    out a
    out b

Previously, pre(1) would buffer its output if it wasn't a terminal, and I didn't
want to add the proprietary -u flag to the sed(1) call.  Now we use stdbuf(1)
instead, and get the same behaviour, besides being much much faster.

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