Avoid creating (and subsequently using) fake opt_*.h files
when the modules are built with the kernel and hence actual
opt_*.h files are available in ${KERNBUILDDIR}. Fix a few
small bugs that would prevent this from working.
At last the build options of the modules become consistent
with those of the kernel unless the MODULES_WITH_WORLD way
still is used.
Approved by: re (kensmith, scottl)
Requested by: ru
Tested on: amd64 i386 sparc64
Tested by: md5
