Introduce a -E flag that activates just the preprocessor.

Introduce a -E flag that activates just the preprocessor.
Use variable sized string buffers for input and macro processing instead
of the fixed buffers used before.
Many changes in the preprocessor to make it more standards compliant (which
it is still not).

git-svn-id: svn://svn.cc65.org/cc65/trunk@3182 b7a2c559-68d2-44c3-8de9-860c34a00d81