libctf: CTFv4: type opening

The majority of this commit rejigs the core type table opening
code for CTFv4: there are a few ancillary bits it drags in,
indicated below.

The internal definition of a child dict (that may not have type or string
lookups performed in it until ctf_open time) used to be 'has a
cth_parent_name', but since BTF doesn't have one of those at all, we add
an additional check: a dict the first byte of whose strtab is not 0 must
be a child.  (If *either* is true, this is a child dict, which allows for
the possibility of CTF dicts with non-deduplicated strtabs -- thus with
leading \0's -- to exist in future.)

The initial sweep through the type table in init_static_types (to size
the name-table lookup hashes) also now checks for various types which
indicate that this must be a CTF dict, in addition to being adjusted
to cater for new CTFv4 representations of things like forwards.  (At
this early stage, we cannot rely on the functions in ctf-type.c to
abstract over this for us.)

We make some new hashtables for new namespace-like things: datasecs
and type and decl tags.

The main name-population loop in init_static_types_names_internal
takes prefixes into account, looking for the name on the suffix type
(where the name is always found).  LSTRUCT handling is removed (they
no longer exist); ENUM64s, enum forwards, VARs, datasecs, and type
and decl tags get their names suitably populated.  Some buggy code
which tried to populate the name tables for cvr-quals (which are
nameless) was dropped.

We add an extra pass which traverses all datasecs and keeps track of which
datasec each var is instantiated in (if any) in a new ctf_var_datasecs hash
table.  (This uses a number of type-querying functions which don't yet
exist: they'll be added in the upcoming commits.)

We handle the type 0 == void case by pointing the first element of
ctf_txlate at a type read in named "void" (making type 0 an alias to it),
or, if one doesn't exist, creating a new one (outside the type table and dtd
arrays), and pointing type 0 at that.  Since it is numbered 0 and not in the
type table or dtd arrays, it will never be written out at serialization
time, but since it is *present*, libctf consumers who expect the void type
to have an integral definition rather than being a magic number will get
what they expect.

include/ctf-api.h

@@ -541,9 +541,10 @@ extern ctf_id_t ctf_lookup_by_symbol_name (ctf_dict_t *, const char *);
 extern ctf_id_t ctf_symbol_next (ctf_dict_t *, ctf_next_t **,
 				 const char **name, int functions);
 
-/* Look up a type by name: some simple C type parsing is done, but this is by no
-   means comprehensive.  Structures, unions and enums need "struct ", "union "
-   or "enum " on the front, as usual in C.  */
+/* Look up a type or variable by name: some simple C type parsing is done, but
+   this is by no means comprehensive.  Structures, unions and enums need "struct
+   ", "union " or "enum " on the front, as usual in C.  Some prefixes not seen
+   in C exist as well: datasecs use "datasec ". */
 
 extern ctf_id_t ctf_lookup_by_name (ctf_dict_t *, const char *);