Support 'using HOSTNAME' syntax

This provides support for configuring monitrc with hostnames as per documentation:

```
SET MAILSERVER <hostname|ip-address [PORT number] [USERNAME string] [PASSWORD string] [using SSLAUTO|SSLV2|SSLV3|TLSV1|TLSV11|TLSV12] [CERTMD5 checksum]>, ...
                [with TIMEOUT X SECONDS]
                [using HOSTNAME hostname]
```