bfd: Rename Chunk and S3Forced

bfd: Rename Chunk and S3Forced

The direct references in objcopy kind of look like a hack to me, so
I'm calling these symbols internal too.  Certainly they aren't named
and documented as a public BFD symbol today anyway.

So ... give these bfd-internal symbols with external linkage a _bfd_
prefix to avoid collisions in the global symbol namespace.

While at it, give them names that more closely match the corresponding
option name that toggles them.

Also while at it, fix a few related comment typos.

gdb/ChangeLog:
2017-02-17  Pedro Alves  <palves@redhat.com>

	* srec.c (Chunk): Rename to ...
	(_bfd_srec_len): ... this.
	(S3Forced): Rename to ...
	(_bfd_srec_forceS3): ... this.
	* objcopy.c: Adjust all references.