Add a COMMENT describing what each component roughly does.

"make describe" prints the comment.

Requested/inspired by Vincent Schwarzer on rumpkernel-users

branches: 1.3.14; 1.3.32;
Default to -Wno-sign-compare -Wno-pointer-sign for clang.
Push -Wno-array-bounds down to the cases that depend on it.
Selectively disable warnings for 3rd party software or non-trivial
issues to be reviewed later to get clang -Werror to build most of the
tree.

branches: 1.2.2; 1.2.4;
Globally define -Wno-pointer-sign, as it has become a pointless
exercise of "add it to every Makefile individually".

XXX: should autosynchronize with the rest of the kernel buildflags
in sys/conf/Makefile.kern.inc.

branches: 1.1.2; 1.1.4; 1.1.6;
Import read-only part of the NiLFS (v2) implementation for NetBSD. It has been
tested with a DEBUG+DIAGNOSTIC+LOCKDEBUG kernel. To summerise NiLFS, i'll
repeat my posting to tech-kern here:

NiLFS stands for New implementation of Logging File System; LFS done
right they claim :) It is at version 2 now and is being developed by NTT, the
Japanese telecom company and recently put into the linux source tree. See
http://www.nilfs.org. The on-disc format is not completely frozen and i expect
at least one minor revision to come in time.

The benefits of NiLFS are build-in fine-grained checkpointing, persistent
snapshots, multiple mounts and very large file and media support. Every
checkpoint can be transformed into a snapshot and v.v. It is said to perform
very well on flash media since it is not overwriting pieces apart from a
incidental update of the superblock, but that might change. It is accompanied
by a cleaner to clean up the segments and recover lost space.

My work is not a port of the linux code; its a new implementation. Porting the
code would be more work since its very linux oriented and never written to be
ported outside linux. The goal is to be fully interchangable. The code is non
intrusive to other parts of the kernel. It is also very light-weight.

The current state of the code is read-only access to both clean and dirty
NiLFS partitions. On mounting a dirty partition it rolls forward the log to
the last checkpoint. Full read-write support is however planned!

Just as the linux code, mount_nilfs allows for the `head' to be mounted
read/write and allows multiple read-only snapshots/checkpoint mounts next to
it.

By allowing the RW mount at a different snapshot for read-write it should be
possible eventually to revert back to a previous state; i.e. try to upgrade a
system and being able to revert to the exact state prior to the upgrade.

Compared to other FS's its pretty light-weight, suitable for embedded use and
on flash media. The read-only code is currently 17kb object code on
NetBSD/i386. I doubt the read-write code will surpass the 50 or 60. Compared
this to FFS being 156kb, UDF being 84 kb and NFS being 130kb. Run-time memory
usage is most likely not very different from other uses though maybe a bit
higher than FFS.