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.bin/getconf: add some initial tests

Items tested via this commit are:
- Some basic POSIX constants.
- Some valid programming environments with -v.
- Some invalid programming environments via -v.

NOTE: this test makes assumptions about ILP32/LP32 vs LP64 that are
currently not true on all architectures to avoid hardcoding some
architectures in the tests. I'm working on improving getconf(1) to be
more sane about handling ILP32/LP32 vs LP64. Future commits are coming
soon to address this.

MFC after: 2 weeks
Tested with: amd64, i386
Sponsored by: Dell EMC Isilon