Add -Werror to the kernel build.

Add -Werror to the kernel build.

* Add a new 'nowerror' keyword to config(8) for parsing the 'files' files.
  When specified, it will prevent the file from being compiled with
  -Werror.

  Note that files that have their own compile-with rules need "${WERROR}"
  added to get -Werror.

* Adjust all 'files' files accordingly: Set nowerror where we still warn
  (i386 or x86_64) and add "${WERROR}" to compile-with rules where it
  applies.

* Remove explicit -Werror-implicit-function-declaration setting as it is
  now useless (-Wimplicit-function-declaration is already part of -Wall).

You will need an updated config(8) (i.e., one that understands 'nowerror')
for this to work so don't forget to quick/buildworld before recompiling
the kernel. nativekernel users will have to update their host config(8).

NO_WERROR in /etc/make.conf applies here as well and turns off the setting
of -Werror for the kernel, too.

In-discussion-with: corecode, aggelos, and others