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/unaccent/unaccent.c

@@ -384,14 +384,14 @@ unaccent_dict(PG_FUNCTION_ARGS)
 		dictOid = PG_GETARG_OID(0);
 		strArg = 1;
 	}
-	str = PG_GETARG_TEXT_P(strArg);
+	str = PG_GETARG_TEXT_PP(strArg);
 
 	dict = lookup_ts_dictionary_cache(dictOid);
 
 	res = (TSLexeme *) DatumGetPointer(FunctionCall4(&(dict->lexize),
 											 PointerGetDatum(dict->dictData),
-											   PointerGetDatum(VARDATA(str)),
-									  Int32GetDatum(VARSIZE(str) - VARHDRSZ),
+										   PointerGetDatum(VARDATA_ANY(str)),
+									   Int32GetDatum(VARSIZE_ANY_EXHDR(str)),
 													 PointerGetDatum(NULL)));
 
 	PG_FREE_IF_COPY(str, strArg);