Add -fbfroginclude for include/fbfrog/, make -include behave like #include

The -include option now "really" behaves like an #include statement.
It will use the normal #include search rules, and it can be filtered out
if wanted. This allows adding #includes that were missing from the normal
input files, without having to pass them as root files, which can't be
filtered out currently.

-fbfroginclude is added to allow inserting any code when starting the CPP,
just like is needed for include/fbfrog/default.h. File names passed here
are searched in include/fbfrog/ etc. This option is needed because stuff
from include/fbfrog/ (especially default.h) may need special treatment.
It may be needed for proper CPP'ing, but it shouldn't affect the binding.
tests/run.bas: Improve comment