Update sys.dirdeps.mk set default DEP_*

Even at level 0 it is handy to default DEP_*
used by Makefile.depend* to aid the first include if
in a leaf dir.

Reviewed by: stevek

Update dirdeps.mk et al

bmake-20240309 includes updates to dirdeps and meta mode makefiles

sys.dirdeps.mk we do want to override OBJTOP

Add more comments to explain what and why.
Ensure OBJROOT ends in / or - (/ preferred).
Export OBJTOP if level > 0
this avoids the issue with bmake/unit-tests.

Check if we have to override MAKEOBJDIR
and if so, put it into env correctly.

sys.dirdeps.mk use ?= for OBJTOP

This broke bmake's after-import step.

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