[preprocessor] Include stacking

[Nathan Sidwell]

This patch refactors pieces of the include stacking routines.  Again 
from the modules branch, so these happen to provide the right slots to 
make that work, but I think they're good in their own right.

_cpp_find_file excitingly hid an if as the loop-invariant conditional of 
a for.  Refactored that, so mostly just changing indentation.

_should_stack_file was confusing, and I broke it apart into two 
predicates, then called from _stack_file.

Finally I extended the include_type enum to add IT_MAIN, for the main 
file, which means _cpp_stack_include just passes the include type down 
to _cpp_stack_file.

Applying to trunk.

nathan
-- 
Nathan Sidwell
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