security/zeek: Fix build on armv7 and allow running as non-root user

Apply Renato Botelho's fix for the ARCH used in PLIST_SUB (with
some changes). Essentially use uname -m instead of trying to fix
up the ARCH defined by bsd.port.mk (uname -p).

While we're here:

• Convert networks.cfg, node.cfg, and zeekctl.cfg to use @sample
• Use @sample to avoid clobbering site.zeek (oops).
• Remove unnecessary subshell for the post-build-NETMAP-on target.
• Silence the annoying "use ZeekControl.plugin instead of BroControl.plugin" nag.
• Reformat the rc.d script.
• Update the rc.d script to honor the zeek_user rc.d variable and update pkg-message.in with hints on how to run as user zeek.
• Explain how to configure zeek to ignore checksum errors when NIC checksum offloading is in use.
• Make share/zeekctl/scripts owned by user zeek to allow "zeekctl install" when running as user zeek.

PR: 250886
Reported by: garga