Support FreeBSD in writable task

[oddnoc]

• Deployer version: 6.8.0
• Deployment OS: macOS 12.1
• Server OS: FreeBSD 13.0-RELEASE-p5

[demo1] > cd /var/silverstripe/demo1/releases/1 && (getfacl -p public/assets | grep "^user:www:.*w" | wc -l)
[demo1] < [error] getfacl: illegal option -- p
[demo1] < [error] getfacl [-dhnqv] [file ...]
[demo1] <        0

The invocation is at line 86 of writable.php

Man page for FreeBSD's getfacl.