internal: Simplify cInclude()
test runner: Abort if fbfrog returns unexpected exit code

This happens e.g. in case of a crash or CTRL+C. In such cases it's more
useful to abort the test suite.
Preserve unexpanded direct #includes and move them to top

This changes the handling of #include statements. Previously they were
expanded if the file was found, and not expanded if the file was not found.
Now they will be preserved depending on whether the include file content
will be preserved or not.

I.e. #includes for files which fbfrog can find but which will be filtered
out due to -filterout won't be missing anymore.
Remap .h to .bi in #include statements (+ some special handling for CRT)
Remove support for preserving comments/whitespace

Even though it's sometimes nice to see documentation being preserved from
the .h file into the .bi file, I don't think it's worth it.

Most C libraries will have better documentation than that in the .h file,
i.e. a website etc. The .h files aren't that great for documentation
purposes anyways, and the same goes for .bi files. Sometimes the .h file
will contain doxygen stuff, which isn't needed in the .bi anyways, since
the .h already exists for the purpose of generating docs.

Furthermore, preserving comments, whitespace and even formatting nicely is
pretty difficult. It did work more or less, although there were remaining
issues. Removing it makes things easier for fbfrog anyways.
lex: Remove references to the no-longer-existing FB mode (see ca53d4f5)
lex: Remove useless saw_end variables
lex: Solve out lexIdentifyCKeyword()
tk: Add common tkIsEolOrEof() function
ast: Simplify astDump()
cpp: Don't push empty file context when skipping #includes

If a header file is skipped due to #include guard optimization, there's no
need to push a file context or insert TK_BEGININCLUDE/TK_ENDINCLUDE at all.
cpp: Don't report skipped #includes

When skipping an #include due to the include guard, there's no need to
report it, as it doesn't affect the binding at all. Its first occurrence
was already reported anyways.