Curl_ntlm_create_type3_message: Tidied up the use of Curl_gethostname.

Removed the code that striped off the domain name when Curl_gethostname
returned the fully qualified domain name as the function has been
updated to return the un-qualified host name.

Replaced the use of HOSTNAME_MAX as the size of the buffer in the call
to Curl_gethostname with sizeof(host) as this is safer should the buffer
size ever be changed.