libctf: add ctf_ref

libctf: add ctf_ref

This allows you to bump the refcount on a ctf_file_t, so that you can
smuggle it out of iterators which open and close the ctf_file_t for you
around the loop body (like ctf_archive_iter).

You still can't use this to preserve a ctf_file_t for longer than the
lifetime of its containing entity (e.g. ctf_archive).

include/
	* ctf-api.h (ctf_ref): New.
libctf/
	* libctf.ver (ctf_ref): New.
	* ctf-open.c (ctf_ref): Implement it.