Use wrappers of PG_DETOAST_DATUM_PACKED() more.

This makes almost all core code follow the policy introduced in the
previous commit.  Specific decisions:

- Text search support functions with char* and length arguments, such as
  prsstart and lexize, may receive unaligned strings.  I doubt
  maintainers of non-core text search code will notice.

- Use plain VARDATA() on values detoasted or synthesized earlier in the
  same function.  Use VARDATA_ANY() on varlenas sourced outside the
  function, even if they happen to always have four-byte headers.  As an
  exception, retain the universal practice of using VARDATA() on return
  values of SendFunctionCall().

- Retain PG_GETARG_BYTEA_P() in pageinspect.  (Page images are too large
  for a one-byte header, so this misses no optimization.)  Sites that do
  not call get_page_from_raw() typically need the four-byte alignment.

- For now, do not change btree_gist.  Its use of four-byte headers in
  memory is partly entangled with storage of 4-byte headers inside
  GBT_VARKEY, on disk.

- For now, do not change gtrgm_consistent() or gtrgm_distance().  They
  incorporate the varlena header into a cache, and there are multiple
  credible implementation strategies to consider.

src/backend/commands/tsearchcmds.c

@@ -1561,9 +1561,9 @@ serialize_deflist(List *deflist)
 List *
 deserialize_deflist(Datum txt)
 {
-	text	   *in = DatumGetTextP(txt);		/* in case it's toasted */
+	text	   *in = DatumGetTextPP(txt);		/* in case it's toasted */
 	List	   *result = NIL;
-	int			len = VARSIZE(in) - VARHDRSZ;
+	int			len = VARSIZE_ANY_EXHDR(in);
 	char	   *ptr,
 			   *endptr,
 			   *workspace,
@@ -1583,7 +1583,7 @@ deserialize_deflist(Datum txt)
 	ds_state	state = CS_WAITKEY;
 
 	workspace = (char *) palloc(len + 1);		/* certainly enough room */
-	ptr = VARDATA(in);
+	ptr = VARDATA_ANY(in);
 	endptr = ptr + len;
 	for (; ptr < endptr; ptr++)
 	{