Add a COMMENT describing what each component roughly does.

"make describe" prints the comment.

Requested/inspired by Vincent Schwarzer on rumpkernel-users

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 r

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.

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Fix RUMPFSLIBS etc. As collateral, rename nullfs/umapfs to match
the vfsname, i.e. to "null" and "umap".