Filter certain control chars from db_qry_parse

That depends on the answer to the previous question as to whether it is legitimate to include low ascii control characters in a string.

Given that these characters can be legitimately represented within the database, I think we have to accept that they are valid.

If Postgresql can happily allow us to insert and select \x06 (for instance), it seems a bit spurious to disallow \x00-05 on the strength of the parser using them internally when parsing the query.
```
[DEV]=# \d ascii
   Table "public.ascii"
 Column | Type | Modifiers 
--------+------+-----------
 test   | text | 
```
```
nscp 115> set test [binary format c* "0x06"]
nscp 116>  db_dml "insert into ascii values('$test')"
nscp 118> db_1row { select test as output  from ascii }

nscp 121> url_encode $output
%06
```

Will look at alternatives.