libctf, link: redo cu-mapping handling

libctf, link: redo cu-mapping handling

Now a bunch of stuff that doesn't apply to ld or any normal use of
libctf, piled into one commit so that it's easier to ignore.

The cu-mapping machinery associates incoming compilation unit names with
outgoing names of CTF dictionaries that should correspond to them, for
non-gdb CTF consumers that would like to group multiple TUs into a
single child dict if conflicting types are found in it (the existing use
case is one kernel module, one child CTF dict, even if the kernel module
is composed of multiple CUs).

The upcoming deduplicator needs to track not only the mapping from
incoming CU name to outgoing dict name, but the inverse mapping from
outgoing dict name to incoming CU name, so it can work over every CTF
dict we might see in the output and link into it.

So rejig the ctf-link machinery to do that.  Simultaneously (because
they are closely associated and were written at the same time), we add a
new CTF_LINK_EMPTY_CU_MAPPINGS flag to ctf_link, which tells the
ctf_link machinery to create empty child dicts for each outgoing CU
mapping even if no CUs that correspond to it exist in the link.  This is
a bit (OK, quite a lot) of a waste of space, but some existing consumers
require it.  (Nobody else should use it.)

Its value is not consecutive with existing CTF_LINK flag values because
we're about to add more flags that are conceptually closer to the
existing ones than this one is.

include/
	* ctf-api.h (CTF_LINK_EMPTY_CU_MAPPINGS): New.

libctf/
	* ctf-impl.h (ctf_file_t): Improve comments.
	<ctf_link_cu_mapping>: Split into...
	<ctf_link_in_cu_mapping>: ... this...
	<ctf_link_out_cu_mapping>: ... and this.
	* ctf-create.c (ctf_serialize): Adjust.
	* ctf-open.c (ctf_file_close): Likewise.
	* ctf-link.c (ctf_create_per_cu): Look things up in the
	in_cu_mapping instead of the cu_mapping.
	(ctf_link_add_cu_mapping): The deduplicating link will define
	what happens if many FROMs share a TO.
	(ctf_link_add_cu_mapping): Create in_cu_mapping and
	out_cu_mapping. Do not create ctf_link_outputs here any more, or
	create per-CU dicts here: they are already created when needed.
	(ctf_link_one_variable): Log a debug message if we skip a
	variable due to its type being concealed in a CU-mapped link.
	(This is probably too common a case to make into a warning.)
	(ctf_link): Create empty per-CU dicts if requested.