clean up uses of ISC_R_NOMEMORY

the isc_mem allocation functions can no longer fail; as a result,
ISC_R_NOMEMORY is now rarely used: only when an external library
such as libjson-c or libfstrm could return NULL. (even in
these cases, arguably we should assert rather than returning
ISC_R_NOMEMORY.)

code and comments that mentioned ISC_R_NOMEMORY have been
cleaned up, and the following functions have been changed to
type void, since (in most cases) the only value they could
return was ISC_R_SUCCESS:

- dns_dns64_create()
- dns_dyndb_create()
- dns_ipkeylist_resize()
- dns_kasp_create()
- dns_kasp_key_create()
- dns_keystore_create()
- dns_order_create()
- dns_order_add()
- dns_peerlist_new()
- dns_tkeyctx_create()
- dns_view_create()
- dns_zone_setorigin()
- dns_zone_setfile()
- dns_zone_setstream()
- dns_zone_getdbtype()
- dns_zone_setjournal()
- dns_zone_setkeydirectory()
- isc_lex_openstream()
- isc_portset_create()
- isc_symtab_create()

(the exception is dns_view_create(), which could have returned
other error codes in the event of a crypto library failure when
calling isc_file_sanitize(), but that should be a RUNTIME_CHECK
anyway.)

lib/dns/sdlz.c

@@ -1024,10 +1024,6 @@ modrdataset(dns_db_t *db, dns_dbnode_t *node, dns_dbversion_t *version,
 	}
 
 	rdatastr = isc_buffer_base(buffer);
-	if (rdatastr == NULL) {
-		result = ISC_R_NOMEMORY;
-		goto cleanup;
-	}
 	rdatastr[isc_buffer_usedlength(buffer) - 1] = 0;
 
 	MAYBE_LOCK(sdlz->dlzimp);