Curl needs the abillity to set a custom user agent with nominatim

If no user agent is set, it would be possible to get a php notice inside the result.

see https://github.com/twain47/Nominatim/blob/master/lib/log.php#L45

If not set the content of the nominatim response is:
```
string(1769) "
Notice: Undefined index: HTTP_USER_AGENT in /home/xxx/Nominatim-2.2.0/lib/log.php on line 28



Rue de ParisGambettaNîmesNimesLanguedoc-Roussillon30900fr
10Rue GambettaMassif-ArmoricainPôle de proximité Nantes-LoireNantesNantesPays de la Loire44000fr"
```