wg-quick: pass on # comments to {Pre,Post}{Up,Down}

wg-quick: pass on # comments to {Pre,Post}{Up,Down}

Currently commands in {Pre,Post}{Up,Down} are stripped of everything
starting with the first #, even if the # is escaped or in a string. This
patch leaves comment interpretation to the shell, as it can presumably
already handle the difference between comments and escaped #.

Signed-off-by: Robyn Kosching <robyn@kosching.me>
[Jason: massage commit message, port to other platforms]
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>