ignore frequent partial search terms

Adds word counts from a full planet to the word table. There is a
new configuration option CONST_Max_Word_Frequency which allows to
take into account the word count: the value that was set on import
is used to determine if a word is added to the search_name table.
The value during runtime determines if a single term should be
used for partial search or simply be ignored.

settings/settings.php

@@ -5,6 +5,7 @@
 	// General settings
 	@define('CONST_Debug', false);
 	@define('CONST_Database_DSN', 'pgsql://@/nominatim');
+	@define('CONST_Max_Word_Frequency', '50000');
 
 	// Paths
 	@define('CONST_Postgresql_Version', '9.1');