[CFT] WITH_META_MODE: Working incremental build

Buildworld/Buildkernel now supports a working incremental build.  Using
-DNO_CLEAN is not needed as it is the default with it (there is no
-DCLEAN for it).  The normal -DNO_CLEAN feature is broken and risky
since it does not track a ton of the dependencies in the build such as
the compiler, other build tools, csu/crt, CFLAGS, build commands, mk
files, etc.

Note that originally WITH_META_MODE initiated a new build system but
that was renamed to WITH_DIRDEPS_BUILD and is something different entirely.

The way that "meta mode" works is to enable the bmake "meta mode"
feature that tracks the build command for each target and uses the
filemon(4) [1] module to track all files read/written/executed for
building target.  If any of these change (see src.conf(5) WITH_META_MODE
for details) then the target will be rebuilt.  This means that even
changes to CFLAGS or the cross compiler will be detected and cause
rebuilds.  This is quite an aggressive system but it works and removes
the need for cleaning any of the tree.  In this mode .depend.* files
(WITH_FAST_DEPEND) are not generated since they are mostly redundant
with filemon(4)'s tracking.

An example in how powerful this is, is that I have today been testing
buildworld with the in-tree clang and also doing builds on the same
objdir with CROSS_TOOLCHAIN=amd64-gcc.  Everything using a compiler
would rebuild and anything not using a compiler would not rebuild.  No
cleaning needed.


To use this you must either add WITH_META_MODE=yes to your environment
or add it into /etc/src-env.conf (not /etc/src.conf or /etc/make.conf).
You will also need to load the filemon(4) module with 'kldload filemon'.

The build will show far less information than the normal build but will
otherwise follow the same flow as buildworld.  If you wish to see the
full output as before you can use -DNO_SILENT or add NO_SILENT=yes to
your /etc/src-env.conf.  If a build error occurs you can view the .meta
file for the target to see what the build command was.  For example if
you see 'Building /usr/obj/root/git/freebsd/world32/root/git/freebsd
/lib/libdwarf/dwarf_arange.So' then see the file
/usr/obj/root/git/freebsd/world32/root/git/freebsd/lib/libdwarf/dwarf_arange.So.meta.

If you wish to see why something is being rebuilt you can pass the -dM
flag to make to see meta mode debug information.


TODO:
- There is currently a bug with library linking that can cause the next
build to consider libraries out-of-date when they are not.  It causes a
chain reaction of relinking but not rebuilds.
- Improve WORLDTMP populating such that it is not done redundantly.
- It's possible rescue/rescue has some quirks still.


[1] Over the years filemon(4) was quite unstable, insecure, and slow. A
lot of work has gone into it over the past 6 months in each of these
areas.  After r300893 it should be fine for this use.

-- 
Regards,
Bryan Drewery