Fixing tag routing

This is an improved and complete fix of #4971. Copying the testing instructions to here:

#### Steps to reproduce the issue

Install Joomla (Test English (GB) Sample Data), for example
From the backend set the configuration ‘Search Engine Friendly URLs’ = ‘YES’
Use the url: index.php?option=com_tags&view=tag&layout=list&id[0]=3 (not sef url into a sef one. The id can have another value and NO Itemid has to be set)

You will have 3 times the error:
Notice: Array to string conversion in /libraries/cms/router/site.php on line 465

The proposal to fix is to replace
 $id = $query['id'];
 with
 $id = (int) $query['id'];
 which seems coherent with the test line 80

#### Expected result
No error

#### Actual result
Notice: Array to string conversion in /libraries/cms/router/site.php on line 465

#### System information (as much as possible)
Joomla 3.3.6

I further cleaned up the router. Basically it should work the same as before, except that that notice is gone. And we have quite a bit less code. :wink: