Enable TCP Fast Open on FreeBSD for role dns

Summary:
While FreeBSD 14 kernel is built by default with support for
TCP Fast Open (option TCP_RFC7413), it's only enabled on boot
for clients, not for servers.

As Knot uses TCP Fast Open, we need to explicitly enable it.

Solves the following issue:

Sep  3 18:35:18 dns-001 knot[81075]: warning:
failed to enable TCP Fast Open on 178.32.70.109@53 (unknown system error)

References:

• tcp(4)

Test Plan:

• sysctl logic tested on WindRiver

Reviewers: DorianWinty

Reviewed By: DorianWinty

Differential Revision: https://devcentral.nasqueron.org/D3655