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/trigger.c

@@ -1673,13 +1673,13 @@ RelationBuildTriggers(Relation relation)
 			bytea	   *val;
 			char	   *p;
 
-			val = DatumGetByteaP(fastgetattr(htup,
-											 Anum_pg_trigger_tgargs,
-											 tgrel->rd_att, &isnull));
+			val = DatumGetByteaPP(fastgetattr(htup,
+											  Anum_pg_trigger_tgargs,
+											  tgrel->rd_att, &isnull));
 			if (isnull)
 				elog(ERROR, "tgargs is null in trigger for relation \"%s\"",
 					 RelationGetRelationName(relation));
-			p = (char *) VARDATA(val);
+			p = (char *) VARDATA_ANY(val);
 			build->tgargs = (char **) palloc(build->tgnargs * sizeof(char *));
 			for (i = 0; i < build->tgnargs; i++)
 			{