libctf: move string deduplication into ctf-archive

libctf: move string deduplication into ctf-archive

This means that any archive containing dicts can get its strings dedupped
together, rather than only those that are ctf_linked.

(For now, we are still constrained to ctf_linked archives, since fixing that
requires further changes to ctf_dedup_strings: but this gives us the first
half of what is necessary.)

libctf/
	* ctf-link.c (ctf_link_write): Move string dedup into...
	* ctf-archive.c (ctf_arc_preserialize): ... this new function.
	(ctf_arc_write_fd): Call it.