This reverts commit befc47822966, as it caused various CI build errors,
and we never merged the prerequisite commit fe815331bb40 ("build:
provide a default WARNS for all in-tree builds"). The latter is rather
intrusive, so I will avoid it.

Direct commit to stable/12 since the CI bots are broken.

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.

(cherry picked from commit 7cc42f6d25ef2e19059d088fa7d4853fe9afefb5)

and finish the job. ncurses is now the only Makefile in the tree that
uses it since it wasn't a simple mechanical change, and will be
addressed in a future commit.

Set WARNS to 6.

MFC after:	1 week

OK'ed by:	core

any fake value.

allows this tool to compile again. Albeit, now to test a new malloc
implementation one has to install the new libc which may have bad
consequences (i.e. if the new malloc implementation were buggy).

Add logic to workaround malloc's current behaviour of returning an
invalid non-NULL pointer for 0 byte allocation requests; this prevents the
tool from coring during the NOPS loop.

Add $FreeBSD$ tags.