Remove $FreeBSD$: one-line sh pattern

Remove /^\s*#[#!]?\s*\$FreeBSD\$.*$\n/

Move cron.d/at to usr.bin/at/

This helps with pkgbase as it tags this as a config file so it is handled as
such

Approved by: allanjude (mentor)
Sponsored by: Essen Hackathon
Differential Revision: https://reviews.freebsd.org/D16673