Don't build EISA by default anymore. Remove from i386 GENERIC and
create an option that defaults to "no" on all platforms to not build
the EISA bits.

Discussed on: arch@
Add -x to the removal of trees to prevent crossing mount points to
limit the damage from errored out runs and failure to unmount loopback
mounts properly.
An abstracted down version of the NanoBSD config files as used by
FreeNAS and the BSDRP folks.
More properly account for free/reserved segments to avoid deadlock or
worse when filling up a device and then trying to erase files to make
space. Without enough space, you can't do that. Also, ensure that the
metadata writes don't generate ENOSPC. They will be retried later
since the buffers are still dirty...

Submitted by: mjg@
Take out the hack to write -1's to non-NAND. Always do a BIO_DELETE on
the ranges we want to erase. This is nicer to SSDs that want TRIMs
anyway.
Convert NO_MANCOMPRESSED to MK_MANCOMPRESSED.
Separate out enabling building clang and/or gcc for the system and
building clang and/or gcc as the bootstrap compiler. Normally, the
default compiler is used. WITH_CLANG_BOOTSTRAP and/or
WITH_GCC_BOOTSTRAP will enable building these compilers as part
bootstrap phase.  WITH/WITHOUT_CLANG_IS_CC controls which compiler is
used by default for the bootstrap phase, as well as which compiler is
installed as cc.  buildworld now successfully completes building the
cross compiler with WITHOUT_CLANG=t and WITHOUT_GCC=t and produces a
built system with neither of these included.

Similarlly, MK_BINUTILS_BOOTSTRAP controls whether binutils is built
during this phase.

WITHOUT_CROSS_COMPILER will now force MK_BINUTILS_BOOTSTRAP=no,
MK_CLANG_BOOTSTRAP=no and MK_GCC_BOOTSTRAP=no.

BOOTSTRAP_COMPILER was considered, but rejected, since pc98 needs both
clang and gcc to bootstrap still. It should be revisisted in the
future if this requirement goes away. Values should be gcc, clang or
none. It could also be a list.

The odd interaction with Xfoo cross/external tools needs work, but
is beyond the scope of this change as well.
Copy bsd.own.mk to bsd.opts.mk and separate out the option setting
code from the rest. Include bsd.opts.mk in bsd.own.mk to preserve
current behavior. Future revisions will replace the inclusion of
bsd.own.mk elsewhere with bsd.opts.mk or a more appropriate new
file that's still being finalized.
Allow setting of MK_FOO by anybody. There are too many edge cases that
we're preventing now with this policy. However, these edge cases
should be rare and all that set MK_FOO directly.
WITH_FOO and WITHOUT_FOO both being defined now result in the
non-default behavior happening silently. Users needing determinism
here fall into the edge case exception for MK_FOO setting.
Move the generic part of bsd.opts.mk into bsd.mkopts.mk to allow for
the WITH/WITHOUT_FOO -> MK_FOO={yes,no} stuff to be used elsewhere.
Retire compatibility YES_HESIOD. If you haven't upgraded in the last
decade to WITH_HESIOD, your are in for a rude surprise...
Document YES_HESIOD disappearing.