Remove $FreeBSD$: one-line sh pattern

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

Move more DIRDEPS_BUILD settings

local.meta.sys.env.mk should now concern itself only with
META_MODE.

Move any DIRDEPS_BUILD settings to local.sys.dirdeps.env.mk

Move DIRDEPS_BUILD settings to sys.dirdeps.mk

When originally implemented, there was no distinction between
DIRDEPS_BUILD and META_MODE, they were one and the same.

META_MODE however is useful by itself, but since meta.sys.mk
had lots of settings related to DIRDEPS_BUILD its use was limited
to DIRDEPS_BUILD.

Move (most) DIRDEPS_BUILD related items to sys.dirdeps.mk
so that meta.sys.mk can be used for just META_MODE.
There is of course some bluring of the lines, so settings remain
in meta.sys.mk

Add MK_META_ERROR_TARGET to enable the META_MODE .ERROR target
independent of DIRDEPS_BUILD, it copies failed .meta files to ${SB}/error
to make it easier to identify the cause of build failures.

Since sys.dirdeps.mk should be included first, most of
local.meta.sys.mk becomes local.sys.dirdeps.mk
and some other bits need to move to local.sys.dirdeps.env.mk

Also fix dirdeps.mk to not add CURDIR to DIRDEPS when it is SRCTOP.

Reviewed by: bdrewery
Differential Revision: https://reviews.freebsd.org/D40053

Set MK_host_egacy=yes earlier

If we need to set MK_host_egacy=yes we might need to check it
in local.toolchain.mk so set it earlier in local.meta.sys.env.mk

Leave the default MK_host_egacy?=no in local.sys.mk so it can
be tested in Makefiles without concern for build mode.

Set UPDATE_DEPDFILE=NO for showconfig

Exporting UPDATE_DEPDFILE=NO from makeman didn't work,
back to setting it in local.meta.sys.env.mk

Extract some of local.meta.sys.mk to local.meta.sys.env.mk

Latest meta.sys.mk follows sys.mk in including local.meta.sys.env.mk
and local.meta.sys.mk late, but we have things we need done early
so move them to local.meta.sys.env.mk