Generated docs and include files

Generated docs and include files

bfd/doc/chew.c extracts documentation from source code comments
annotated with keywords, and generates much of bfd.h and libbfd.h from
those same comments.  The docs have suffered from people (me too)
adding things like CODE_FRAGMENT to the source to put code into bfd.h
without realising that CODE_FRAGMENT also puts @example around said
code into the docs.  So we have random senseless things in the docs.
This patch fixes that problem (well, the senseless things from
CODE_FRAGMENT), moves most of the code out of bfd-in.h, and improves a
few chew.c features.  libbfd.h now automatically gets ATTRIBUTE_HIDDEN
prototypes, and indentation in bfd.h and libbfd.h is better.