libctf, archive: stop ctf_arc_bufopen triggering crazy unmaps

libctf, archive: stop ctf_arc_bufopen triggering crazy unmaps

The archive machinery mmap()s its archives when possible: so it arranges
to do appropriately-sized unmaps by recording the unmap length in the
ctfa_magic value and unmapping that.

This brilliant (horrible) trick works less well when ctf_arc_bufopen is
called with an existing buffer (which might be a readonly mapping).
ctf_arc_bufopen always returns a ctf_archive_t wrapper, so record in
there the necessity to not unmap anything when a bufopen'ed archive is
closed again.

libctf/
	* ctf-impl.h (struct ctf_archive_internal)
	<ctfi_unmap_on_close>: New.
	(ctf_new_archive_internal): Adjust.
	* ctf-archive.c (ctf_new_archive_internal): Likewise.
	Initialize ctfi_unmap_on_close.  Adjust error path.
	(ctf_arc_bufopen): Adjust ctf_new_archive_internal call
	(unmap_on_close is 0).
	(ctf_arc_close): Only unmap if ctfi_unmap_on_close.
	* ctf-open-bfd.c (ctf_fdopen): Adjust.