Aid the user in setting up filterchains by better logmessages

The builtin filterchain writer module requires to know the name of the writer module to send the data to. I didn't find any documentation about how the writers names were created; some of them are dynamicaly generated depending on the configuration. 
This patch outputs the name of all configured writers right next to the errormessage of being unable to choose one so the user can succeed in generating his config without source reading and documentation nightmares.
the return value from malloc should be checked, I also find `calloc` to be a better match for this. 
another nitpick, even though `sizeof(char **) == sizeof(char *)`, you're really looking for `sizeof(char *)`
A good way to avoid falling in this trap is to use the following idiom:

```c
char **keys = NULL;

if ((keys = calloc(n, sizeof(*keys))) == NULL)
   /* error out */
```