libctf: add CU-mapping machinery

Once the deduplicator is capable of actually detecting conflicting types
with the same name (i.e., not yet) we will place such conflicting types,
and types that depend on them, into CTF dictionaries that are the child
of the main dictionary we usually emit: currently, this will lead to the
.ctf section becoming a CTF archive rather than a single dictionary,
with the default-named archive member (_CTF_SECTION, or NULL) being the
main shared dictionary with most of the types in it.

By default, the sections are named after the compilation unit they come
from (complete path and all), with the cuname field in the CTF header
providing further evidence of the name without requiring the caller to
engage in tiresome parsing.  But some callers may not wish the mapping
from input CU to output sub-dictionary to be purely CU-based.

The machinery here allows this to be freely changed, in two ways:

 - callers can call ctf_link_add_cu_mapping to specify that a single
   input compilation unit should have its types placed in some other CU
   if they conflict: the CU will always be created, even if empty, so
   the consuming program can depend on its existence.  You can map
   multiple input CUs to one output CU to force all their types to be
   merged together: if some of *those* types conflict, the behaviour is
   currently unspecified (the new deduplicator will specify it).

 - callers can call ctf_link_set_memb_name_changer to provide a function
   which is passed every CTF sub-dictionary name in turn (including
   _CTF_SECTION) and can return a new name, or NULL if no change is
   desired.  The mapping from input to output names should not map two
   input names to the same output name: if this happens, the two are not
   merged but will result in an archive with two members with the same
   name (technically valid, but it's hard to access the second
   same-named member: you have to do an iteration over archive members).

This is used by the kernel's ctfarchive machinery (not yet upstream) to
encode CTF under member names like {module name}.ctf rather than
.ctf.CU, but it is anticipated that other large projects may wish to
have their own storage for CTF outside of .ctf sections and may wish to
have new naming schemes that suit their special-purpose consumers.

New in v3.
v4: check for strdup failure.
v5: fix tabdamage.

include/
	* ctf-api.h (ctf_link_add_cu_mapping): New.
	(ctf_link_memb_name_changer_f): New.
	(ctf_link_set_memb_name_changer): New.

libctf/
	* ctf-impl.h (ctf_file_t) <ctf_link_cu_mappping>: New.
	<ctf_link_memb_name_changer>: Likewise.
	<ctf_link_memb_name_changer_arg>: Likewise.
	* ctf-create.c (ctf_update): Update accordingly.
	* ctf-open.c (ctf_file_close): Likewise.
	* ctf-link.c (ctf_create_per_cu): Apply the cu mapping.
	(ctf_link_add_cu_mapping): New.
	(ctf_link_set_memb_name_changer): Likewise.
	(ctf_change_parent_name): New.
	(ctf_name_list_accum_cb_arg_t) <dynames>: New, storage for names
	allocated by the caller's ctf_link_memb_name_changer.
	<ndynames>: Likewise.
	(ctf_accumulate_archive_names): Call the ctf_link_memb_name_changer.
	(ctf_link_write): Likewise (for _CTF_SECTION only): also call
	ctf_change_parent_name.  Free any resulting names.

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