Mk/bsd.port.mk: fix install-missing-packages without a tty

install-missing-packages runs xargs with -o, which reopens stdin from
/dev/tty so that pkg can prompt interactively. In automated or batch
runs there is no controlling terminal, so xargs fails with "can't open
/dev/tty: Device not configured".

Make the -o flag conditional on BATCH: keep the interactive pkg prompt
outside BATCH, and use pkg install -yA (auto-confirm) inside BATCH.

PR: 267416
Reported by: patmaddox@FreeBSD.org