Remove $FreeBSD$: one-line sh pattern

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

Do a sweep and remove most WARNS=6 settings

Repeating the default WARNS here makes it slightly more difficult to
experiment with default WARNS changes, e.g. if we did something absolutely
bananas and introduced a WARNS=7 and wanted to try lifting the default to
that.

Drop most of them; there is one in the blake2 kernel module, but I suspect
it should be dropped -- the default WARNS in the rest of the build doesn't
currently apply to kernel modules, and I haven't put too much thought into
whether it makes sense to make it so.

usr.sbin: normalize paths using SRCTOP-relative paths or :H when possible

This simplifies make logic/output

MFC after: 1 month
Sponsored by: Dell EMC Isilon

Add main() for iovctl and hook iovctl into build

Differential Revision: https://reviews.freebsd.org/D87
Reviewed by: jhb
MFC after: 1 month
Sponsored by: Sandvine Inc.