libctf: create, types: variables and datasecs (REVIEW NEEDED)

This is an area of significant difference from CTFv3.  The API changes
significantly, with quite a few additions to allow creation and querying of
these new datasec entities:

-typedef int ctf_variable_f (const char *name, ctf_id_t type, void *arg);
+typedef int ctf_variable_f (ctf_dict_t *, const char *name, ctf_id_t type,
+			    void *arg);
+typedef int ctf_datasec_var_f (ctf_dict_t *fp, ctf_id_t type, size_t offset,
+			       size_t datasec_size, void *arg);

+/* Search a datasec for a variable covering a given offset.
+
+   Errors with ECTF_NODATASEC if not found.  */
+
+ctf_id_t ctf_datasec_var_offset (ctf_dict_t *fp, ctf_id_t datasec,
+				 uint32_t offset);
+
+/* Return the datasec that a given variable appears in, or ECTF_NODATASEC if
+   none.  */
+
+ctf_id_t ctf_variable_datasec (ctf_dict_t *fp, ctf_id_t var);

+int ctf_datasec_var_iter (ctf_dict_t *, ctf_id_t, ctf_datasec_var_f *,
+			  void *);
+ctf_id_t ctf_datasec_var_next (ctf_dict_t *, ctf_id_t, ctf_next_t **,
+			       size_t *size, size_t *offset);

-int ctf_add_variable (ctf_dict_t *, const char *, ctf_id_t);
+/* ctf_add_variable adds variables to no datasec at all;
+   ctf_add_section_variable adds them to the given datasec, or to no datasec at
+   all if the datasec is NULL.  */
+
+ctf_id_t ctf_add_variable (ctf_dict_t *, const char *, int linkage, ctf_id_t);
+ctf_id_t ctf_add_section_variable (ctf_dict_t *, uint32_t,
+				   const char *datasec, const char *name,
+				   int linkage, ctf_id_t type,
+				   size_t size, size_t offset);

We tie datasecs quite closely to variables at addition (and, as should
become clear later, dedup) time: you never create datasecs, you only create
variables *in* datasecs, and the datasec springs into existence when you do
so: datasecs are always found in the same dict as the variables they contain
(the variables are never in the parent if the datasec is in a child or
anything).  We keep track of the variable->datasec mapping in
ctf_var_datasecs (populating it at addition and open time), to allow
ctf_variable_datasec to work at reasonable speed.  (But, as yet, there are
no tests of this function at all.)

The datasecs are created unsorted (to avoid variable addition becoming
O(n^2)) and sorted at serialization time, and when ctf_datasec_var_offset is
invoked.

We reuse the natural-alignment code from struct addition to get a plausible
offset in datasecs if an alignment of -1 is specified: maybe this is
unnecessary now (it was originally added when ctf_add_variable added
variables to a "default datasec", while now it just leaves them out of
all datasecs, like externs are).

One constraint of this is that we currently prohibit the addition of
nonrepresentable-typed variables, because we can't tell what their natural
alignment is: if we dropped the whole "align" and just required everyone
adding a variable to a datasec to specify an offset, we could drop that
restriction. WDYT?

One additional caveat: right now, ctf_lookup_variable() looks up the type of
a variable (because when it was invented, variables were not entities in
themselves that you could look up).  This name is confusing as hell as a
result.  It might be less confusing to make it return the CTF_K_VAR, but
that would be awful to adapt callers to, since both are represented with
ctf_id_t's, so the compiler wouldn't warn about the needed change at all...
I've vacillated on this three or four times now.

libctf/libctf.ver

@@ -104,6 +104,8 @@ LIBCTF_2.0 {
 	ctf_type_iter_all;
 	ctf_variable_iter;
 	ctf_variable_next;
+	ctf_datasec_iter;
+	ctf_datasec_next;
 
 	ctf_next_create;
 	ctf_next_destroy;
@@ -124,19 +126,20 @@ LIBCTF_2.0 {
 	ctf_add_type;
 	ctf_add_typedef;
 	ctf_add_restrict;
+	ctf_add_section_variable;
 	ctf_add_slice;
 	ctf_add_struct;
 	ctf_add_union;
 	ctf_add_struct_sized;
 	ctf_add_union_sized;
 	ctf_add_unknown;
+	ctf_add_variable;
 	ctf_add_volatile;
 
 	ctf_add_enumerator;
 	ctf_add_member;
 	ctf_add_member_offset;
 	ctf_add_member_encoded;
-	ctf_add_variable;
 	ctf_add_member_bitfield;
 
 	ctf_set_array;