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.

contrib/pg_prewarm/pg_prewarm.c

@@ -79,7 +79,7 @@ pg_prewarm(PG_FUNCTION_ARGS)
 		ereport(ERROR,
 				(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
 				 (errmsg("prewarm type cannot be null"))));
-	type = PG_GETARG_TEXT_P(1);
+	type = PG_GETARG_TEXT_PP(1);
 	ttype = text_to_cstring(type);
 	if (strcmp(ttype, "prefetch") == 0)
 		ptype = PREWARM_PREFETCH;
@@ -99,7 +99,7 @@ pg_prewarm(PG_FUNCTION_ARGS)
 		ereport(ERROR,
 				(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
 				 (errmsg("relation fork cannot be null"))));
-	forkName = PG_GETARG_TEXT_P(2);
+	forkName = PG_GETARG_TEXT_PP(2);
 	forkString = text_to_cstring(forkName);
 	forkNumber = forkname_to_number(forkString);