Define bfd_realloc(ptr,0) to return allocated memory, whilst bfd_realloc_or_free...

Define bfd_realloc(ptr,0) to return allocated memory, whilst bfd_realloc_or_free(ptr,0) frees ptr.

	* libbfd.c (bfd_malloc): Provide some documenation.  Treat a size
	of 0 as 1.
	(bfd_realloc): Likewise.
	(bfd_zmalloc): Likewise.
	(bfd_realloc_or_free): Provide some documentation.  Treat a size
	of 0 as a request to free.
	* libbfd-in.h (bfd_realloc): Remove prototype.
	(bfd_realloc_or_free): Remove prototype.
	(bfd_zmalloc): Remove prototype.
	* libbfd.h: Regenerate.