CWARNFLAGS must be added to after including kern.mk, otherwise all the
warnings set in kern.mk are ignored. This is why the
-Wmissing-prototypes warnings were not seen previously.
While here, drop -Wno-pointer-sign as it is doesn't seem to be needed,
but add -Wno-cast-qual to silence warnings for various casts of const
pointers to non-const pointers.
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