sec->alloced and freeing section contents

This modifies _bfd_elf_free_cached_info to unmap/free section
contents.  To do that we need to *not* free sections where contents
are bfd_alloc'd or point to constant strings or somesuch.  I've chosen
to implement this be adding another flag to struct bfd_section,
"alloced" to say the section contents can't be freed.  Most of the
patch is about setting that flag in many places.