wg-quick: use addconf instead of setconf

wg-quick: use addconf instead of setconf

The example in the man page at some point changed:

-    \fBPostUp = wg set %i private-key <(pass WireGuard/private-keys/%i)\fP
+    \fBPreUp = wg set %i private-key <(pass WireGuard/private-keys/%i)\fP

This is actually wrong because PreUp is followed by set_config(), which
calls `wg setconf`, which in turn deletes the private key from the
interface because it is missing from the configuration. Replacing this
with `wg addconf` is safe to do because the interface is newly created.

Suggested-by: Matthias Dressel <code@deadcode.eu>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>